Anote Tong says Kiribati is facing a growing threat from rising sea levels. A 50cm increase would inundate most of the archipelago.
"During the last high tide, the water was coming over," the former Kiribati president told the Global Citizen Now conference in Perth.
"Half a metre above that would mean that most of the land area will be overtopped.
"That is what our future is."
Mr Tong said Kiribati urgently needed to start "radical adaptation measures" if it was to survive.
"Raise all the islands; in the past I've spoken about floating islands," he said.
"We need the technical people, engineers, to come up with credible engineering solutions."
Mr Tong acknowledged the high cost of the project and said securing funding had become an emergency situation.
He also called out Australia's action on climate change while continuing to export coal and other fossil fuels.
"On the one hand you are cutting your emissions," he said.
"(But) it's not your local emissions; it's what you export."
In further criticism, the Pacific Elders' Voice chair took aim at the attitude of some Australians in the wake of the recent pre-COP31 Tuvalu meeting.
"I've read the social comments here in Australia ... saying it's a waste of time," he said.
"The commentary was brutal."
Mr Tong said it was disappointing that people who clearly didn't understand what was happening in the Pacific region were commenting about the meeting.
"Please understand what it is, and this is part of the story," he said.
Mr Tong served as the President of the Republic of Kiribati from 2003 to 2016.
Kiribati is a group of 32 low-lying atolls with an average height above sea level of just two metres.
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