An Australian man is missing at sea along with two Fijian co-workers after a fishing boat with four people on board sank off the coast of Papua New Guinea.

The Australian Maritime Safety Authority says it's helping PNG authorities with aerial searches off the coast of Port Moresby for the three men who have been missing since Saturday evening.

Fijian media have named two of the men as Praneel Nand and Kapil Chand.

It is understood four men were on a fishing trip when the boat capsized.

One man was rescued and hospitalised with hypothermia after a rescue helicopter spotted him.

The three missing men are all employees of accountancy firm KPMG and a spokesperson for the company says they remain hopeful the men will be found alive despite the treacherous conditions.

"We are in close contact with the families, the Department of Foreign Affairs and Trade and authorities," the spokesperson said.

The DFAT is providing consular assistance for the Australian man's family.