Sharon Granites has not been seen since her family put her to bed at a town camp near Alice Springs late on Saturday night, sparking a desperate search for the girl and her suspected abductor.
Recently-released prisoner Jefferson Lewis, 47, is accused of taking Sharon from an Old Timers town camp residence, triggering a manhunt that entered its fifth day on Thursday.
Northern Territory Police Commissioner Martin Dole said Sharon's family had reported the child was non-verbal and often communicated with her hands.
He told ABC Radio Alice Springs that may complicate matters because she would be unable to communicate her needs to Lewis and "she's not able to communicate with searchers".
He compared the resources used in the investigation to those deployed during the search for British tourist Peter Falconio, after his infamous outback murder by Bradley Murdoch.
Based on advice, Sharon's chances of survival are "starting to become quite concerning for people", Mr Dole said.
The commissioner remained adamant Lewis was being helped by others and was still in the Alice Springs area.
"The evidence is the lack of information, it is the absence of any sightings ... that is highly unusual," he said.
Police on Wednesday said there were people in the community who "absolutely know" where Lewis was and urged them to contact police.
They also revealed that a man's T-shirt, a child's underwear and a doona were found at a designated crime scene near the town camp and they were being forensically examined.
The possibility of a sexual assault being committed was "on the table", investigators said.
Bess Nungarrayi Price, a former NT government community services minister and member of Sharon's extended family, called on Lewis to surrender to police and disclose where the girl was being kept.
"What the family are saying is we want to ask him, or for him to tell whoever he's been staying with, if she's safe and where they can find her, if he has got her," she told the ABC.
Assistant Commissioner Peter Malley said the chance of finding Sharon alive reduced the longer the search dragged on.
"She may have come to harm, but we still think she's alive and our number one mission is to find her safe and well," he said earlier.
He warned that anyone sheltering Lewis would face charges.
Lewis was sentenced to 64 months in prison, between 2016 and 2025, for offences including aggravated assaults, breaching domestic violence orders, bail and resisting police.
Scores of police officers, emergency service personnel, defence force members, Aboriginal trackers and community volunteers have been scouring tough desert terrain to find Sharon and her alleged abductor.
Helicopters, drones, horses, dogs, all-terrain vehicles and motorbikes have been used in the search.
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