Papua New Guinea's Prime Minister Peter O'Neill has invoked the nation's death penalty in response to a brutal attack on an international group of trekkers.

Mr O'Neill on Wednesday condemned "in the strongest possible terms" the murder of two porters by bandits and their attack on Australian and New Zealand trekkers on the Black Cat trail in Morobe Province.

"These are appalling crimes, and they attract the death penalty under laws passed by the National Parliament since the last election," Mr O'Neill said in a statement.

"I make no apology whatsoever for the death penalty being the punishment available to be applied for such crimes," he said.

The group were attacked on Tuesday afternoon by six men armed with two guns, machetes and a spear.

Mr O'Neill said there was no possible excuse for the brutal crimes against visitors to PNG and those acting as their guides.

"At a time when we are seeking to increase tourism these crimes are an obvious setback - but we must not let them deter tourists and travellers generally visiting Papua New Guinea, and our own people helping visitors in their travels," he said.

"I express my sympathy to the families of the guides who were murdered, and the victims who sustained injury. I hope all make a speedy recovery."