Police have revealed what led them to charge a man with the murder of nine-year-old Charlise Mutten, whose body was found in a barrel by a NSW river.
Charlise was reported missing from Mount Wilson on Friday and a search had been carried out in bushland for the past five days.
Her mother's partner, Justin Stein, 31, was arrested at a home in inner Sydney last night and charged with murder.
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A search in the vicinity of the Colo River, about an hour from the Blue Mountains, located human remains last night.
The body of a child, believed to be Charlise, was found in a barrel on public property.
The remains are "consistent with the missing nine-year-old", Acting Commissioner Karen Webb has confirmed.
About 8.30pm yesterday, detectives arrested Mr Stein at a unit in Riley Street, Surry Hills, in inner Sydney.
He was charged with murder and appeared briefly in court today.
He was refused bail to next appear in court on March 18.
Charlise had been staying with her mother during the school holidays.
Her mother is now in hospital after suffering a medical episode after reporting the girl missing.
How police tracked the accused
There were a number of "anomalies in relation to movement data", NSW Police Deputy Commissioner David Hudson said today.
"Through electronic means, through GPS tracking, through CCTV tracking, we were able to establish certain facts in relation to the vehicle he was driving and those movements and also certain suspicious behaviour which escalated our scrutiny in relation to his potential involvement," he said.
Police believe the little girl was killed at Mount Wilson sometime between Tuesday night and Wednesday last week.
Police will allege Mr Stein bought up to 100 kilograms of sandbags from Bunnings at Marsden Park on Thursday before fuelling his boat at a nearby BP.
He's then accused of driving to several locations, including Windsor boat ramp, where divers have focused their search, but the boat was inoperable.
His last stop was Colo, where he allegedly placed the barrel on the edge of the river.
The following morning, the girl's mother reported her missing before Mr Stein spoke to police at Penrith.
Officers towed his car on a nearby street as he did so.
Mr Stein allegedly gave many different versions of events to police, but detectives tracked his phone and collected CCTV, which led them to the barrel.
Deputy Commissioner Hudson also said the girl's cause of death will not be known until a post-mortem is undertaken.
"Until the post-mortem is conducted, we are still uncertain of what exactly happened to her," Deputy Commissioner Hudson said.
"What we are sure of is that the accused that we charged with murder was responsible for, firstly, placing her in the barrel, and then disposing of that barrel in the bushland where she was located."
Acting Commissioner Webb said the police investigation would continue.
"There is still a lot to determine… it's now in the hands of the coroner, cause of death and a number of other lines of inquiry," she said.
"The search will remain ongoing to look for any clues to help us identify the cause of the death."
She urged anybody with information to come forward.
"There are still many elements of this investigation that we have to work through to determine exactly what happened from the time the young girl was reported missing up until last night when those remains were found," Acting Commissioner Webb said.
'Horrific' end after massive police effort
Deputy Commissioner Hudson thanked the police, emergency service workers, and volunteers, who searched a large area of the Blue Mountains and surrounds for five days in challenging weather conditions.
"Obviously we start off these searches with a lot of hope but, unfortunately, we always have in the back of our minds the worst and that's why lessons from previous investigations have taught us that we don't just need to conduct the missing persons search but also, in parallel to that, an investigative search, which revealed remains last night," Deputy Commissioner Hudson said.
"Again, the police involved – we've had over 200 investigators from State Crime Command assisting north-west region detectives on the investigative side of this.
"That has been quite a laborious task in analysing GPS data, analysing CCTV footage and leading us to search the location we searched last night."
NSW Deputy Premier and Minister for Police Paul Toole described the outcome as "horrific".
"We've had hundreds of police, RFS, and SES and volunteers on the ground over the last five days," he said.
"I want to thank them again for their efforts, because they have worked through torrential conditions.
"We've seen pouring rain over that time. They never gave up hope. People across this country have been hoping and praying for a good outcome.
"Today's news is not what we wanted to hear.
"I mean, this is horrible. This is horrific. This is shocking.
"I know that many people are distressed by what they're hearing today."
