Ms Cremer, 31, was reported missing in June 2023 after she failed to return from a bushwalk at Philosopher Falls, in Tasmania's rugged northwest.
Police called off an initial two-week search following advice she could not have survived so long in freezing winter conditions.
But her family and friends refused to give up, arranging a group of volunteers led by private investigator Ken Gamble to continue the search last year.
The group of about 30 people searched for five days before calling off the search, except for a smaller group of volunteers who carried on.
That effort culminated in the discovery of human remains in late January, which sparked a renewed search by police.
On Sunday, a search team consisting of police and wilderness paramedics made out for the area, where they found further remains and a car key.
The operation focused on a 350-metre stretch of River Arthur, about 2km from Philosopher Falls, near Cradle Mountain.
While forensic testing of the remains is ongoing, police confirmed the key was Ms Cremer's after successfully using it to unlock her vehicle.
Reflecting on the latest developments, Mr Gamble said investigators are closing in on an answer to the two-and-a-half year old question.
"I think we can now make a fairly good determination about what happened to Celine," Mr Gamble told AAP.
"We know that she got lost. We know that she lost her phone. We know that she's possibly drowned in the Arthur River."
While Ms Cremer's loved ones won't have complete closure yet, in his view the mystery had largely been put to bed.
"It's given me closure as an investigator," he told AAP.
"I'm very satisfied we've done everything we can to solve the case."
He likened the discovery of Ms Cremer's car keys to finding "a needle in a haystack."
"To be finding a phone and then keys and then bones and all these things, it's quite miraculous, really," Mr Gamble said.
Police are awaiting the outcome of a review of the most recent search effort to determine any future operations.
On Wednesday, Commander Nathan Johnston said the breakthrough suggested investigations were moving in the right direction and was consistent with the theory Ms Cremer became lost during a bushwalk.
Mr Gamble praised the efforts of police, claiming a review of their log books showed just how extensive even their early search efforts were.
"I was astounded at the amount of work they had done, the amount of searching they'd done in the background," he said.
"I don't think any criticism could be levelled against the police whatsoever. They did a very thorough job."
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