An Australian professor and three people from the University of PNG have been captured in the Southern Highlands, with the criminals demanding a ransom for their safe release.
Posting a message of support on social media, a friend of one of the hostages said their "heart bleeds not know how my beautiful and gentle (friend) and her colleagues are doing in the middle of the jungles".
"In the midst of this dark and ugly situation, we are very appreciative of the support of our local and national leaders and authorities working through this situation," they wrote.
Friends and colleagues also expressed sorrow and anger at the situation in social media comments, and feared "jungle pirates" were getting bolder and more confident.
AAP has decided not to name the captives or point to any identifying information.
Papua New Guinea police remain locked in negotiations with the criminals through missionaries acting as intermediaries.
Police Commissioner David Manning says all options remain on the table to secure the hostages' release, with their safety being the top priority.
"Our specialised security force personnel will use whatever means necessary against the criminals, up to and including the use of lethal force, in order to provide for the safety and security of the people being held," Mr Manning said in a statement on Tuesday morning.
The police commissioner said he believed the hostages were taken by chance by the armed men who took them into the bush and demanded a ransom.
"However, we are offering the abductors a way out. They can release their captives and they will be treated fairly through the criminal justice system," Mr Manning said.
"But failure to comply and resisting arrest could cost these criminals their lives."
Air support has been approved and the military was on standby in surrounding regions as of Monday.
The four hostages had been conducting field research in the Southern Highlands.
ANU associate professor Sinclair Dinnen said reports suggested the kidnapping was likely opportunistic and there wasn't any evidence pointing to an uptick in hostage-taking in PNG.
"There have been incidents, but it's still fairly unusual," he told AAP.
Prof Dinnen said if the captors were motivated by financial gain, then Westerners made better targets because of their wealth compared to locals.
He says that international media coverage also puts pressure on the government to resolve the situation quickly.
PNG Prime Minister James Marape initially said five hostages remained on Monday but the police commissioner said only four had been taken.
There have been no reports of anyone being freed.
Australia has a no-ransom policy and opposition foreign affairs spokesman Simon Birmingham said it's not in the nation's interest to be handing over ransoms unnecessarily.
"We need to be working hand in glove with the government of Papua New Guinea, with their law enforcement authorities to try to ensure the swift and safe release of these individuals," he told Sky News on Tuesday.
"Sometimes that means they will have to work quietly behind the scenes to try to secure the best outcome."
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