PORT MORESBY - Rescue workers are trying to get fresh water and medical supplies to thousands of villagers sheltering from an erupting volcano on the northern Papua New Guinea island of Manam.

Mount Iabu, in PNG's Madang province, started throwing up lava and ash on Sunday, prompting authorities to advise nearly 4000 villagers to move to safer ground.

"The falling ash has contaminated what little water supplies there are on the island," said a spokeswoman for PNG Prime Minister Sir Michael Somare. "Gardens have been buried, banana trees destroyed and there is only one trade store on the affected part of the island which is fast running out of food."

She said most people were in church when the volcano erupted, which may have saved them from falling fist-sized rocks and hot ash.

"These were big rocks and could have done some serious damage if they'd hit people but most people were in church and remained there under shelter."

The province's acting administrator, Robert Yass, said about 2000 people were seeking shelter at a Catholic mission while thousands of others had moved to the island's south-east side to escape falling ash.

- AAP