A super-charged El Nino could leave 4.7 million people across the Pacific without food and water.
A new report by Oxfam says Papua New Guinea is already bearing the brunt with up to three million people at risk of crop failures and having to scale down food intake to one meal a day because of frosts and drought.
Vanuatu, Fiji, the Solomon Islands, Samoa and Tonga are also experiencing worsening drought while Kiribati and Tuvalu will likely see intense rain causing flooding and higher sea levels, the report says.
Oxfam said early intervention was vital in order to head off a humanitarian crisis.
"The warning bells are deafening," humanitarian manager Meg Quartermaine said.
Drought conditions are also taking hold across of parts of Africa, Asia, the Caribbean and Central and South America.
Oxfam estimates that worldwide 40-50 million people will face hunger, disease and water shortages by early next year.
The last major El Nino event in 1997-98 led to widespread deaths, displacement of communities and outbreaks of disease.
The federal government has already allocated $9 million in aid to PNG and $4 million to Somalia.
But Oxfam is urging Australia to do more.
