Indonesia's devastating 'mud volcano' could keep spewing for the next 30 years, filling the equivalent of 50 Olympic-size swimming pools every day, a top Australian expert warned on Thursday.

Curtin University of Technology's doctor Mark Tingay, who has just returned from the disaster site in East Java, said about 100,000 people remained under threat from subsidence three years after the volcano first erupted.

"In effect, the whole region around the vent hole is sinking by about two to five centimetres each day due to the rising mud level, causing more damage to suburban villages and triggering frequent bursts of flammable gas around homes," he said, according to a Geological Society of Australia statement.

Tingay added that damage caused by the mud, which has been devouring land and homes in Sidoarjo district since May 2, 2006, was estimated at about 4.9 billion dollars.

The volcano has buried 12 villages, killed 13 people, displaced more than 42,000 residents and wiped out 800 hectares (1,977 acres) of densely populated farming and industrial land.

He said the volcano could produce enough scalding mud to fill Sydney Harbour twice over in the next 30 years but admitted the time-scale was only an estimate.

"The high flow-rate may only continue for two to three years, or it might continue for hundreds of years," Tingay said.

"And like other mud volcanoes, Lusi will probably be in existence for thousands of years, even if its flow-rate subsides," he added.

Australian oil and gas giant Santos, which was drilling in the area when the volcano erupted, by September had declared previsions of just 88.5 million dollars to cover the clean-up cost.

In December, Santos exited the project and said it would pay an Indonesian firm 22.5 million US dollars "to support long-term mud management efforts" at the site.