Some leaders were more willing to consider at least partial relocation. President Atone Tong of Kiribati, the country at most severe risk, said residents acknowledged their landmass, and population, would shrink.
"The question then is, what do we do? Do we hang onto it? Or [do we leave]? It's a rational strategy to consider both. We've got over 100,000 people, and there's no way all these people will be able to migrate."
For Kiribati, there was a sense of urgency on climate change at the forum.
Mr McCully said there was a risk of the debate occurring at an "academic level", and the central Pacific country required immediate, practical action. New Zealand had focused on moving it off petrol-powered electricity and helped with management of oceans and fisheries.
Mr Tong said his Government had bought land in Fiji to resettle people and was focused on upskilling the young population so they could find jobs in other countries.
Climate plans 50 shades of beige
Pacific leaders arguably made louder statements with their shirts than their climate change promises on the final day of the Pacific Islands Forum.
The traditional wearing of silly shirts capped the end of the official programme in the Marshall Islands yesterday, as 15 Pacific leaders headed to Eneko Island for the leaders' retreat.
In an open-air pavilion the leaders pulled together the roadmap for Pacific development over the next year.
Mr Key said there had been no tangible change to New Zealand's emissions target of 5 per cent below 1990 levels, but pointed to continuing commitments such as helping Pacific nations use more renewable energy - a hugely ambitious goal of moving the Pacific from 5 per cent renewable energy to 50 per cent.
President Tommy Remengesau of Palau, who will host the forum next year, promised to pick up the momentum on climate change.
The communique also condemned the use of chemical weapons in Syria - at the suggestion of Mr Key, noted the positive progress Fiji had made in moving to free and fair elections next year and supported further action to resolve nuclear test-related damages owed to the Marshalls.
Low-level truth
Kiribati
Population: 100,000
Average height above sea level: less than 2m (highest point 81m)
Tuvalu
Population: 11,000
Average height above sea level: 2m (highest point 5m)
Marshall Islands
Population 55,000
Average height above sea level: 2m (highest points 10m)