Charter company Hevilift has confirmed an Australian pilot, his co-pilot and one passenger, both of whom were PNG nationals, were killed when the Hevilift Twin Otter aircraft carrying seven passengers went down near the capital Port Moresby.

The six PNG national survivors, one in a critical condition, are being treated at a Port Moresby hospital.

The flight was travelling from Woitape in Goilala district to Port Moresby on Saturday morning but crashed about seven kilometres south of the PNG Defence Force Supply and Ordinance Barracks at Goldie.

The chief executive of PNG's Accident Investigation Commission, David Inau, told the ABC the cause of the crash was unclear.

"It's too early to say until [investigators] come back from the accident site," he said.

Australian High Commission officials in Port Moresby are understood to be seeking further information about the crash and the identity of the Australian pilot.

Hevilift said in a statement it was "cooperating with all relevant authorities to determine the cause" of the crash and would be "conducting a thorough investigation".

"Hevilift's thoughts and sympathy go to the families of those who have died in the crash and the company will continue to provide every assistance to the survivors."

The incident is the second fatal crash for Hevilift in recent years, after two Australians and a New Zealander were killed in July 2012 when their helicopter crashed while flying from a drill rig to the mainland.

In 2011 a plane crash near Madang killed 28 of the 32 people on board.