Kiribati's sudden withdrawal from the Pacific Islands Forum without wider political discussion by the government is out of order, the country's opposition leader and its first president Sir Ieremia Tabai says.

The Kiribati government gave the Pacific Islands Forum secretary general a letter on Saturday, that said it was withdrawing from the body immediately.

The latest forum summit is now underway in Suva.

Tabai was stunned to hear from RNZ Pacific that the country had left the forum, and he said the public had not been told.

"It's complete news to me. Explain to the people why that decision has been made. It's not on - in a system like ours he's accountable to all the people of Kiribati and he should explain himself, but he has not.

"It's simply not good enough."

Tabai said he could understand the clash with Tuesday's national day, but at the very least a minister should have been sent to the forum in the president's place.