Australia's small island neighbours in the South Pacific face an enormous bill to protect their coastal buildings and infrastructure from the impacts of climate change and extreme weather, one they are unlikely to afford.

Research out of the University of New England has for the first time sought to determine the extent of coastal buildings at risk across 12 South Pacific island nations, including Vanuatu and Samoa, putting the cost of replacing those in harm's way at almost $24 billion.

The Marshall Islands hit by king tides. It is one of the South Pacific nations researchers say are facing unaffordable costs to protect their coastal buildings. Credit:Benedict D. Yamamura

The study also found that more than half (57 per cent) of the buildings assessed across the 12 countries were within 500 metres of the coast, making them susceptible to damage under current climate conditions and to the more intense extreme weather, rising sea-levels and storm surges projected with further global warming.

The paper, published in the journal Nature Climate Change on Tuesday, is a part of a larger project the researchers carried out for Australia's Department of Foreign Affairs and Trade to help it prioritise its aid effort in the South Pacific. There are 23 countries and territories in the region, with the 12 chosen for the study having the most complete data.