Police believe the pilot, Tasmanian man Bob Bramley, did not survive the crash of the Air Frontier twin-engined Beechcraft Baron aircraft after it took off from Gove Airport at Nhulunbuy in the Northern Territory.
The plane was due to land at the remote Indigenous community of Numbulwar at 11.50am on Tuesday but never arrived.
A search began after a witness on a vessel reported seeing the plane crash into the sea near Isle Woodah in the Gulf of Carpentaria.
Police and Aboriginal ranger vessels, fixed-wing aircraft, a helicopter and police divers have been deployed in the search.
The divers found aircraft debris on the sea floor near the reported crash site, including components believed to be part of the plane's airframe, foam and sections of seat cushions.
Sonar searches have also been conducted in the area and police air observers have scanned areas identified as having the highest probability of detection based on drift modelling and previous search results.
Search co-ordinator Acting Sergeant Nathan Conelius said on Friday the decision had been made to scale back aerial searching following three days of extensive operations.
Expert advice was that the likelihood of locating the pilot through continued broad-area surface and aerial searching was now extremely low, he said in a statement.
"Police continue to liaise closely with the pilot's family and are keeping them informed of developments and the ongoing search effort," he said.
The pilot's family said the death of Mr Bramley was the "great loss in our lives", describing him as courageous and inspirational,Â
As a 16-year-old in 2019, Mr Bramley circumnavigated Australia in a small plane to raise awareness of youth suicide, saying he was motivated to do so after almost losing two very close friends.
He went on to found the youth suicide prevention initiative Surviving Turbulence and was featured on Australian Story in 2021 for his record-breaking solo flight.
Air Frontier has grounded all five of its Beechcraft Baron aircraft pending advice from authorities.
The Australian Transport Safety Bureau has initiated a crash investigation.
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