Papua New Guinea's parliament has voted to extend a ban on votes of no confidence to 30 months into a government's five-year term.

PNG's opposition backed away from supporting the law, saying it will take the matter to the Supreme Court to undo it.

The bill passed into law in PNG's single House of Parliament on Tuesday with a vote of 90 to 14, well above the 74 votes needed to make the constitutional change.

Prime Minister Peter O'Neill said the law - which extends the ban from the previous 18 months - will give government more time to do its job instead of shoring up numbers to defend itself politically.

"This is not about increasing my power," Mr O'Neill said.

"It is about locking in to place the long-term political stability our nation, especially our people, have been denied for so long.

"It is a reform our people have been calling for - and most certainly our business sector, and our investors, have been calling for."

Debate on the law was gagged following a statement by Mr O'Neill and an opposition member.

Opposition leader Belden Namah and most of his 15-strong team voted against the bill, just four months after announcing their support for the measure, and he said he will follow through with court action.

One member of the opposition voted with the government.

"By this afternoon or tomorrow morning, we will file a Supreme Court reference because we have cited that in that particular amendment, it wasn't done properly," he said, telling reporters the law was not documented properly.

The man who just eight months ago was Mr O'Neill's deputy said the law was not about stability, but about concentrating power in the PM.

"What has happened now - those who voted for the bill - have become insignificant," he told journalists following the vote.

"They have weakened their own bargaining and negotiating power for projects in their districts. It's as simple as that.

"They have put themselves into voluntary exile ... from now on, Peter O'Neill is the most powerful man in the country."

PNG has lost two prime ministers to votes of no confidence since independence in 1975 - Sir Michael Somare and Paias Wingti.

However, the threat of a vote has been a significant part of the Pacific nation's political makeup, causing its prime ministers to become entrenched in money politics to keep colleagues on side.