Australia's first day of summer kicks off with one side of the country sweltering and another shivering, with extreme weather ushering in a change of seasons.

Today Perth is heading for a maximum temperature of 37 degrees, putting the city's hottest December 1 record of 37.4 degrees recorded in 1979 within sight.

In stark contrast, the mercury in Melbourne is forecast to climb to only 16 degrees, but that could turn out to be even cooler, with a chance the city's record summer low from 1987 of 15.5 degrees being broken.

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Meanwhile, snow has fallen at the highest elevations of three states – Tasmania, Victoria and New South Wales, Weatherzone said.

Thredbo in the Snowy Mountains of NSW, has settled snow down to about 1600 metres.

One of the Thredbo ski cams confirmed a dusting of the white stuff.

Two powerful but contrasting weather patterns are behind this bizarre start of summer, reports Weatherzone.

Over in Western Australia, an often-seen trough is moving down the coast, pulling hot air from the state's interior to Perth and south-west regions.

The north-west of the WA is also forecast for scorching conditions over the next two day.

The Bureau of Meteorology has issued a severe heatwave warning for the Kimberley region.

Meanwhile, following an unusually chilly late spring, the wintry conditions in south-eastern Australia are forecast to linger into early summer.

https://twitter.com/BOM_au/status/1995223712241520780?ref_src=twsrc%5Etfw

The frigid weather in the Victorian Alps and Tasmanian Highlands has brought steady snowfalls, and they're likely to continue today.

Melbourne residents will be keeping their winter woollies handy, with thick cloud and showers, making it feel more like winter's first day than the opening of summer.

Looking ahead, Perth can expect some relief from the heat when the mercury falls to a maximum of 28 degrees tomorrow.

For Melbourne, warmer conditions should arrive by Wednesday, with a top of 27 degrees expected.

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Four men, including an independent journalist and a former swimming coach, have been charged over their alleged involvement in an international satanic child sex abuse material ring.

After investigations by Strike Force Constantine, police uncovered a Sydney-based alleged paedophile network possessing, distributing and facilitating child abuse material involving "symbols and rituals related to satanism and the occult".

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Police allege thousands of files of child abuse material were uncovered in initial investigations of electronic devices seized.

On Thursday, six search warrants were conducted across Sydney, including in Waterloo, Ultimo and Malabar.

Landon Germanotta-Mills, 26, was arrested in Waterloo in Sydney's southern suburbs and is alleged to be the leader of the group.

Germanotta-Mills is the founder of the Underground Media Network, which describes itself as an independent investigative journalism platform.

The platform on its website claims to investigate issues in "child protection… [and] public interest harm".

"We centre survivors, whistleblowers, and community voices — especially where institutions have failed to act," the website reads.

Germanotta-Mills was taken to Mascot Police Station, where he was charged with multiple offences, including seven counts of using a carriage service to access child abuse material.

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At a unit block in Malabar in Sydney's eastern suburbs, three men aged 46, 42 and 39 were arrested and taken to Maroubra Police Station.

One of those arrested was Mark Sendecky, a former swimming coach.

Sendecky was charged with possessing child abuse data using a carriage service, accessing child abuse material, and possessing child abuse material.

Speaking to media, Detective Superintendent Jayne Doherty said the material was "abhorrent" and outlined some of the alleged activities of the group.

"[They engaged] in conversations and the sharing of material which depicted child abuse and the torture of children," she said.

"Police will allege in court that the initial examination of these devices has discovered thousands of videos depicting the abuse of live children between the ages of babies and 12 years of age.

"Amongst these deplorable images were also a number of videos depicting the sexual abuse of animals."

Police are not alleging the men created the material.

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Doherty said the material was particularly appalling due to the sinister connotations behind it.

"These ones were particularly devastating in [that] they used symbols and rituals around it in the discussions about abusing children," she said.

"It had a ritualistic overview."

Police charged the oldest man with multiple offences, including using a carriage service to make child abuse available.

The 39-year-old was also charged with multiple offences, including possessing child abuse material and possessing bestiality material.

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Doherty confirmed the men were allegedly part of an international group dealing with "satanic child abuse material", but said she police could not confirm the specific identity of any of the children involved, or if any of them are from Australia.

"We've not identified an actual child, but the images are of real children," she said.

There is also no confirmation the men had any immediate access to children, but Doherty said the risk to the community was still high, hence the arrests being made.

"There are no specific children they had access to, but they were put in our community and we were concerned for the children within the community," she said.

All men were refused bail and appeared in court on Friday.

They will next appear at Downing Centre Local Court on January 27 and January 29 next year.

Support is available from the National Sexual Assault, Domestic and Family Violence Counselling Service at 1800RESPECT (1800 737 732).

Kids Helpline 1800 55 1800.

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A longer-than-normal influenza season is having an impact on hospitals in New South Wales, health officials have said.

In the week ending on November 16, more than 3100 cases of the flu were identified around the state, with 370 people presenting to emergency departments with flu-like symptoms.

NSW Health also reported that Healthdirect, an online health platform, has seen over 50,000 calls from people experiencing respiratory symptoms during this flu season.

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It was the second week in a row that cases climbed, and were similar to the amount of cases seen at the start of June, where flu season is considered to be near its peak.

It is a sign the flu is remaining prevalent around the states, and experts admit this is unusual so close to summer.

"When we compare this respiratory illness season to others, we are experiencing a longer tail, which is a concern," Chief Health Official Kerry Chant said.

"Usually by September or early October, influenza levels have tapered to quite low levels from the peak, which traditionally occurs during winter. 

"But we're well into November and notifications are increasing."

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Whilst officials insist the state's health system is managing to deal with the surprise number of flu cases, they are urging people in the state to take precautions, especially with the busy Christmas and New Year period approaching.

​"My worry, and the worry of our health experts, is that we're seeing an increase at a time of year where people are rightfully getting together to celebrate, but the last gift we want to be giving each other is a dose of influenza," Health Minister Ryan Park said.

He said there is one way residents can protect themself year-round from a bout of the flu.

"By getting the influenza vaccination and staying home when we're unwell, we can all do our bit to support health staff and protect loved ones from serious illness," he said.

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Millions of Aussies have no idea they could lose thousands of dollars in benefits by December 31, a study claims.

The extras benefit limits of many health funds reset on New Year's Day, meaning any unclaimed health insurance extras such as dental, optical or physio will be wiped out.

This means 15 million Aussies with health insurance only have a month to use up these extras, and a survey by Money.com.au revealed up to 40 per cent of Aussies didn't even realise they have unclaimed extras that could disappear by the end of this month.

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"Australians are effectively walking away from thousands of dollars in value every year," Chris Whitelaw from Money.com.au warned.

"Many people don't realise those benefits vanish overnight on December 31. It's a classic 'use it or lose it' scenario."

Around 31 per cent of Aussies say they still have some benefits left unused on their plan, and 15 per cent admitted they have used none or very little of their extras.

The survey found older Australians were more likely to be uncertain about whether they have unclaimed extras, with Whitelaw explaining attitudes may impact this trend," he said.

"This means they're less aware of their annual limits."

However, younger Aussies are more likely not to use any of their extras benefits at all, costing them the most.

Around 19 per cent of Millennials say they have used none or a very small amount of their extras this year.

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Dozens of climate change protesters have been arrested amid further disruptions at the world's largest coal port.

More coal ships were blocked from entering the Port of Newcastle today as Rising Tide protesters on kayaks and other small vessels called for taxes on fossil fuel export profits and an end to new coal developments.

Police said 141 people, including 18 children, believed to be associated with the multi-day event had been arrested by 4.30pm today.

Of the 123 adults arrested, most were charged with various offences under the Crimes and Marine Safety while inquiries continued into two women arrested on board a ship. 

Footage from today showed a banner reading "phase out coal and gas" being unfurled by two people hanging from the side of the vessel.

The Port Authority of NSW reportedly said four ships were disrupted on Sunday, including two not linked to coal. 

"Our intention was to stop coal ships from entering or leaving the Port of Newcastle and we've succeeded in doing this today," Rising Tide organiser Zack Schofield said.

"By continuing to approve new coal and gas mines, the Albanese government is failing to protect us from the worst impacts of climate change and when the government fails us, the people must rise."

Police alleged one boat, driven by a 26-year-old protester collided with a police jet ski that gave chase after the larger vessel refused to stop as it entered an exclusion zone. 

About 11.20am, a fight "allegedly involving attendees of the event" was reported at Foreshore Park. 

Police said a 46-year-old man was taken to Calvary Mater Hospital in a stable condition after suffering lacerations to his back, arms, legs, and head, and a 17-year-old boy who intervened was also injured. 

An 18-year-old was charged with two counts of assault occasioning actual bodily harm (DV) and other offences. 

"The NSW Police Force recognises and supports the rights of individuals and groups to exercise their rights of free speech and peaceful assembly," Police said, in a statement.

"However, the priority for NSW Police is always the safety of the wider community and there will be zero tolerance for illegal and dangerous behaviour.

Organisers say they are looking to stall new coal developments and tax fossil fuel export profits.

Newcastle sends 150 million tonnes of coal offshore annually.

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A display pilot has died after two light planes collided in Sydney's south-west.

Reports were made at 11.50am that the two Van's Aircraft RV-7 planes collided near Napperfield Airfield in Wedderburn, near Appin.

One of the planes crashed in nearby bushland.

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The body of the pilot, Eddie Seve, from Sydney's Hills District was found by emergency services.

He was part of the Freedom Formation Display Team, which confirmed its aircraft was involved on Facebook.

The other plane landed safely at the airfield and the pilot, Ollie Geraghty, was uninjured.

Police are investigating, along with the Australian Transport Safety Bureau.

It said the two aircraft had been involved in a formation flight of four that was returning to land at Wedderburn Airport when the collision happened.

"One of the aircraft involved in the collision landed safety but the other aircraft collided with terrain and its pilot was fatally injured," a statement said.

It's the second crash at the airport in a year.