Research team taken hostage in Papua New Guinea© Getty Images
The group was on a study trip in the remote highlands when captured by armed men, according to media reports.
A ransom has been demanded for their release, officials say.
PNG Prime Minister James Marape says authorities are in contact with the kidnappers and hope to have the hostages released “alive and safe”.
A larger group was initially taken captive but some local guides have since been released, according to reports.
It is unclear exactly how many people remain held, but Australian news outlets put the number at four or five. They include an archaeologist from an Australian university and academics and students from PNG.
Mr Marape told reporters that PNG authorities had heard from the Australian man who confirmed the group was alive.
He said there were “running conversations” between officials and the kidnappers, and police and the military were on stand-by to assist.
“But, in the first instance, we want those criminals to release those who are held in captivity,” he said.
“There is no place to run, there is no place to hide.”
Mr Marape did not say when the group was taken and said the situation had been kept “under close wraps because of the sensitivity”.
But he sought to reassure families of the hostages, saying: “I’m confident, I’m optimistic, I’m prayerful that we get… them out.”
Australia’s Department of Foreign Affairs and Trade is yet to comment.
Earlier this month a New Zealand pilot was taken hostage by separatist fighters in Indonesia’s nearby Papua region.