NSW Police have confirmed nine-year-old Charlise Mutten, whose body was found in a barrel on the banks of the Colo River, died after being shot.
Investigators believe the child was killed at a property in the Blue Mountains where she was on holiday with her mother, Kallista Mutten, and her mother's fiance, 31-year-old Justin Stein.
Mr Stein was last week charged with Charlise's murder.
Homicide Squad Commander Danny Doherty said the alleged crime was "unimaginable and egregious".
He said police had released the specific details of Charlise's death because they were appealing to the public for fresh information and there were "significant" gaps in the time line of events.
"It's probably very unpalatable for the community to hear the circumstances of this beautiful nine-year-old girl but it's important for us to do that (release the details) so we get answers for the family," he said.
"Charlise is not here to defend herself. We have to act on her behalf and we want to make sure justice is delivered."
The trio were staying at a luxury estate at Mount Wilson, about two hours west of Sydney.
Police initially said she was last seen on January 13 and reported missing the next morning.
However, they now believe Charlise was killed sometime between late Tuesday, January 11 and the early hours of Wednesday, January 12.
Her disappearance prompted a five-day search through bush surrounding the property.
Police used CCTV and GPS data to track Mr Stein.
It led them to the grim discovery of Charlise's body inside a barrel on the banks of the Colo River, about an hour from the property where the trio was staying.
It is believed a small-calibre firearm was used in the alleged murder.
Police divers and the dog squad are now searching for a firearm at the Mount Wilson property — they have confirmed that the barrel in which Charlise was found was from the property.
Police have released CCTV footage of a red Holden Colorado driving into central Sydney and around the Colo area, and are appealing for public help to address gaps in the time line of events.
"We really need the community's assistance with this," he said.
Police previously revealed Mr Stein unsuccessfully tried to launch a boat in Sydney before driving to the Colo River.
"[Police are] Appealing to anyone who may have seen this red Holden Colorado towing a boat, any suspicious activity or unusual activity around the car and the boat at any of those locations or other locations," Superintendent Doherty said.
Superintendent Doherty said officers had reinterviewed Charlise's mother and her formal statement matched the initial information she gave.
Ms Mutten was hospitalised following Charlise's disappearance, forcing police to wait several days to reinterview her.
She is being treated as a witness as police have evidence she was not at the Mount Wilson property at the time of the alleged murder.
Police are still investigating Mr Stein's alleged motive and why Charlise was left alone with him.
Police will allege that Mr Stein was alone with the nine-year-old in the evening on January 11, when they allege the murder occurred, while her mother was staying at a caravan away from the property.
