A major New South Wales highway has re-opened after black ice closed it for hours amid a late winter snowfall.

The Great Western Highway was closed between Katoomba and Mount Victoria this morning, seeing traffic piled up for hours. Truck drivers even switched off their engines and got out to play in the snow, with no forecast on when the road would re-open.

The Transport Management Centre confirmed that the highway had reopened in both directions at about 10.20am.

READ MORE: Black ice closes major roads as snow falls on south-east

Bells Line of Road, another Blue Mountains thoroughfare, has now also reopened between Lithgow and Bell.

People are advised to give themselves extra travel time. Blue Mountains trains also remain delayed.

Temperatures plummeted 10C in just two hours across Sydney overnight, driven by a sudden southerly wind change bringing a very cold air mass.

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In Katoomba, west of Sydney, it reached -1.5C but felt like -7C, leading to a ground-blanketing snowfall.

More snow fell in parts of Victoria, days out from the beginning of spring on September 1.

Meanwhile, a severe weather warning is in place across New South Wales today.

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Surf and swell is expected to be hazardous along the coast from Byron Bay to the Illawarra.

Residents are advised to stay out of the water.

The cold front that has brought the snow and freezing temperatures has already moved offshore, meaning that the weather won't stick around.

But today, Sydney and much of NSW can prepare for a very cold, gusty day.

In Melbourne the weather is likely to be similar, with a chance of rain in the afternoon.