In a speech in Canberra, Dame Meg Taylor warned that climate change had brought truly desperate times to the Pacific Islands.
Dame Meg urged unity from Australia, saying the region could not afford to have one or two acting in ways that place the wellbeing of the region at risk.
At the Forum Leaders' Summit last week, Australia was the country that tried to water down a commitment to climate change action.
The government of Australian prime minister Scott Morrison was also planning to officially dump emissions reduction targets.
Pacific countries are increasingly reproaching Australia for its inaction on climate change, driving a wedge between Canberra and the rest of the region as it tries to reassert itself.