The Pacific Islands Forum opened with a passionate plea from Kiribati for help staving off rising seas caused by climate change, while discontent festered over Fiji's ban from the summit.

Kiribati President Anote Tong said his low-lying nation was being engulfed as global warming lifted sea levels, forcing it to consider radical solutions such as moving its 100,000-strong population onto man-made floating islands.

The nation, consisting of small coral atolls, urgently needed the world to take action on climate change to prevent it sinking beneath the waves, Tong said.

"I've always appealed to the morality of humanity and I think we are challenging humanity to answer that question," he told reporters shortly after the annual PIF summit formally opened in Auckland.

Some villagers in the country have already been forced to relocate and Tong said if the situation continued to decline the government would consider moving the entire population to artificial islands.

"The concept is that they would be something like oil drilling rigs," he said, adding the plan had been costed at $2 billion.

"The last time I saw the models, I was like 'wow it's like science fiction'," he said.

Tong said such a solution was not Kiribati's preferred option but everything was being considered as the country looked to secure its future.

UN Secretary-General Ban Ki-moon, who travelled to Kiribati this week to see the impact of climate change first-hand, said he made the trip to Auckland to underscore the importance of the issue.

"I am here to continue sounding the global alarm about climate change," he said.

Kiribati's Tong called for Fiji's reinstatement to the 16-nation PIF, saying it explusion two years ago in the wake of a 2006 military coup was achieving nothing.

The forum's dominant powers New Zealand and Australia strongly support the ban, imposed after military strongman Voreqe Bainimarama reneged on a promise to hold elections.

Since taking power, Bainimarama has suspended the constitution, sacked the judiciary, muzzled the media and been accused of human rights abuses.

"Our position has always been very clear, we don't condone what's happened but we disagree on how to deal with it," Tong told reporters.

"We've always taken the position that we should continue to engage (with Fiji)."

He said the country remained part of the "Pacific family" and its people should not be punished for the actions of Bainimarama's regime.

"If you have six children and one is a bad young boy, what do you do with him, kick him out?" Tong said.

"You don't, you never do, because we're a family and we must act like a family."

Asked if he believed other Pacific Islands Forum states shared his view on lifting the Fiji ban, Tong replied: "Yes I do, but they won't say."

New Zealand Prime Minister John Key said it was natural that Pacific leaders felt uncomfortable about Fiji's ban but he believed most of them wanted it to remain in place until there was proof the regime was moving toward democracy.

He dismissed suggestion that New Zealand was attempting to stifle debate on readmitting Fiji by not including the issue on the summit's formal agenda.

"I don't think you can make the case that we are trying to suppress the issue," he said.