Pacific island leaders have agreed to disagree on how to tackle climate change, after Australia and New Zealand blocked a bid from low-lying island nations for a tougher global target.

Problems with negotiations behind closed doors at the Pacific Islands Forum in Port Moresby were evident with the official communique, due Thursday evening, not released until mid-morning Friday.

The 16-nation group consists mainly of small island nations, most of which are susceptible to rising sea levels.

They wanted to send a clear message to climate talks in Paris in December that the average global warming should be limited to 1.5 degrees Celsius (2.7 degrees Fahrenheit) over pre-Industrial Revolution levels.

However, the UN mandate is for no more than a 2.0-degree rise, and the stumbling block at the conference was the refusal of the two regional powers Australia and New Zealand to commit to the lower number.

The carefully worded communique noted the Pacific Island states were among "the most vulnerable and least able to adapt and to respond" to the consequences of climate change.

The Pacific Islands Forum leaders "declare that an increase of 1.5 degrees Celsius would severely exacerbate the particular challenges facing the most vulnerable smaller island states of the Pacific and urge all effort be made to stay within the global temperature goal".

At a press conference ahead of the communique release, Kiribati President Anote Tong said it was an agreement to disagree.

"It's not the best outcome that we would have liked," he said of Australia and New Zealand's refusal to back a figure lower than the UN mandate.

"I think we must respect that. Whether we accept that or not is a different question," Tong said.

Many of the island states are barely two metres above sea level, which leaders said added weight to their lower target.

They faced serious problems "on the frontline" of global warming and were in a very different position to Australia and New Zealand, Tong said.

The forum mostly comprises poor island states and Fiji Prime Minister Voreqe Bainimarama in May called for an end to Australia and New Zealand's "undue influence" on Oceania's largest regional grouping.

Ahead of the meeting the leaders of the group's six smallest members said the upcoming climate talks in Paris were the last chance for the world to reach an agreement that could save their vulnerable island nations.

Tuvalu Prime Minister Enele Sopoaga said he would continue to push for 1.5 degrees to be a legally binding target.

"That's the position of the Forum island countries. It is 1.5 degrees and not 2.0 degrees. We will take it to Paris and battle it out there."

Australian Prime Minister Tony Abbott said Australia would go to Paris with "very ambitious targets" and he believed they could get emissions down.

New Zealand leader John Key said his country and Australia stood by the 2.0 degree target but accepted that low-lying states were particularly vulnerable and they would "seek an even more ambitious target in Paris".