Five Australians accused of illegally landing a plane in Indonesia's sensitive Papua province have been detained in a hotel for a fourth day on Monday, officials say.

The two women and three men were arrested on Friday when they landed at Mopah airport in Papua's Merauke district after a flight from Australia in a light plane.

Provincial immigration official Freddy Manus said the Australians had committed a "serious violation" of Indonesian law and their case had been referred to Jakarta.

"The central government will take a decision on this matter because it's not a simple immigration offence but a serious violation," he said on Monday.

"They are still under tight surveillance from police and immigration officials here in Merauke."

The airport supervisor at Merauke said the Australians were being held in a local hotel pending a decision from Jakarta. "They are still isolated," he said.

The five were identified as pilots William Henry Scott Bloxam and Vera Scott Bloxam, and passengers Hubert Hofer, Karen Burke and Keith Mortimer.

Papua is the most sensitive part of Indonesia due to a low-level separatist insurgency which has been simmering since the 1960s. Journalists are barred from the province without special permits.

Officials said the Australians had no approval to land in Indonesia and no visas, and their passports had expired.

The Australians claimed to be visiting the eastern province on a tourist trip, according to Indonesian officials involved in the investigation.

Australia's Department of Foreign Affairs and Trade (DFAT) confirmed the detention of the Australians, a DFAT spokesman saying they were detained for not having flight, security or immigration approvals.

The Australian embassy in Jakarta was providing consular assistance.