A report by the charity said Pacific Islanders were already feeling the effects of global warming, including food and water shortages, rising cases of malaria and more frequent flooding and storms. Some had already been forced from their homes and the number of displaced people was rising, it warned.
"The Future is Here: Climate Change in the Pacific" predicted that many Pacific Islanders would not be able to relocate within their own countries and would become international refugees.
It urged neighbouring wealthy countries to take urgent action to curb their carbon emissions to prevent a large-scale crisis.
Half of the population of the Pacific live less than 1.5km from the coast and are incredibly vulnerable to sea-level rise and extreme weather. But as well as moving out, the report found that some countries had started adapting to the changing climate.
Fiji is attempting to "climate-proof" its villages by testing salt-resistant varieties of staple foods, planting mangroves and native grasses to halt coastal erosion in order to protect wells from salt water intrusion, and moving homes and community buildings away from vulnerable coastlines.
In the Solomon Islands officials are looking for land to resettle people from low-lying outer atolls, and those living in the outer atolls of the Federated States of Micronesia were also moving to higher ground. The tiny nation of Tuvalu also recently pledged to become carbon neutral by 2020.
Andrew Hewett, Oxfam Australia Executive Director, said it was vital that Australia started working with Pacific governments to plan for the impact of climate change.
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As the wealthiest country in the region and the highest per capita polluter, Australia "must prevent further climate damage to the Pacific by urgently adopting higher targets" - reducing emissions by at least 40 per cent on 1990 levels by 2020 - and urging other developed countries to do the same, the report said.
The Australian government's commitment of $150 million (£75m) to help Pacific Islanders adapt to climate change needed to at least double, it said.
"It would be in Australia's interests to act now because, as the situation worsened, it would be called on to respond to more emergencies in the region," Mr Hewett told the Sydney Morning Herald.
With only months to go until the crucial UN negotiations in Copenhagen in December, Australia needed to show Pacific leaders it was willing to do its fair share to address one of the most pressing challenges in the region, he said.
"People are already leaving their homes because of climate change, with projections that 75 million people in the Asia-Pacific region will be forced to relocate by 2050 if climate change continues unabated. Not all will have the option of relocating within their own country, so it's vital that the Australian Government starts working with Pacific governments to plan for this now."
Pacific leaders will raise the issue of climate change with Kevin Rudd, the Australian prime minister, at the Pacific Islands Forum on Aug 4.