Some of Kiribati's 32 pancake-flat coral atolls, which straddle the equator over 1,350,000 square miles of ocean, are already disappearing beneath the waves.
Most of its 113,000 people are crammed on to Tarawa, the administrative centre, a chain of islets which curve in a horseshoe shape around a lagoon.
"This is the last resort, there's no way out of this one," Mr Tong said.
"Our people will have to move as the tides have reached our homes and villages."
Mr Tong said the plan would be to send a trickle of skilled workers first, so they could merge more easily with the Fijian population and make a positive contribution to that country's economy.
"We don't want 100,000 people from Kiribati coming to Fiji in one go," he told the state-run Fiji One television channel.
"They need to find employment, not as refugees but as immigrant people with skills to offer, people who have a place in the community, people who will not be seen as second-class citizens.
"What we need is the international community to come up with an urgent funding package to deal with that ambition, and the needs of countries like Kiribati."
The land Kiribati wants to buy is understood to be on Vanua Levu, Fiji's second largest island.
Mr Tong's proposal is the latest in an increasingly desperate search for solutions.