The six attackers, armed with machetes, spears and guns, descended on the group Tuesday afternoon at a camping area along the mountainous Black Cat Track in Morobe province on Papua New Guinea's northeast coast, Papua New Guinea national police spokesman Dominic Kakas said.
Eight Australians and one New Zealander were injured, though none of the wounds were life-threatening, Australia's Department of Foreign Affairs and Trade said in a statement.
Two local porters were killed and nine others were hurt, six seriously, Kakas said. The tourists received bruises and lacerations, and one was speared through his leg. No gunshots were reported to have been fired during the attack, Kakas said.
The victims were transported to the Morobe capital of Lae for medical treatment.
Violence is rife throughout Papua New Guinea, an impoverished tribal nation of 7 million north of Australia. A recent escalation in cases of extreme brutality, including beheadings, burnings and gang rapes, prompted the government to expand the list of crimes punishable by death earlier this year.
Still, Tuesday's attack was unusual for the area, a popular hiking destination for travelers, and police were puzzled over the possible motive. Three of the tourists had their passports stolen during the attack, but nothing else of value was taken, Kakas said.
"We are at a loss as to how this has happened," Kakas said. "The Black Cat Track has been going on for many, many years now and there's been not one incident that we know of. ... It really caught us all by surprise here."
In a statement, Police Commissioner Toami Kulunga called the attack "shocking and just unthinkable," and said police were using a helicopter Wednesday to hunt for the assailants in the rugged terrain.
Australia's foreign affairs department warned travelers to avoid the trail until the investigation is complete.
