Protests have spread to a fourth detention centre in Australia housing asylum-seekers, after a spate of suspected arson attacks at three other camps.

Refugee groups said inmates were protesting against a government policy of locking them up - often for several years - while their applications for asylum were processed.

A staff member at a camp in Perth, Western Australia, was treated for facial injuries after a "small uprising" involving about 20 detainees there on Monday.

Baxter centre was only built four months ago

The damage caused by the fires at three other centres over the weekend is expected to mount to more than A$5 million (US$2.8 million).

Immigration officials said four people were arrested after the latest protest in Perth and will appear in court on Tuesday to face charges of assault and resisting arrest.

A series of fires have already destroyed more than 77 rooms at the newly-built Baxter centre in South Australia, Reuters news agency said.

The fires left eight detainees needing treatment for smoke inhalation.

Two small fires were also reported in a toilet block in the notorious Woomera camp, also in South Australia, and firefighters were called early on Monday to blazes in two unoccupied buildings at Western Australia's Port Headland centre.

Two staff there were treated for minor smoke inhalation.

Immigration officials have said the fires were deliberately lit.

But Australian Prime Minister John Howard said the protests would not change his approach to the issue.

"We are not going to change our policy (just) because people set fire to detention centres," he said.

Asylum seekers have held hunger strikes, sewed their lips together, and broken out of detention centres in earlier protests.

Woomera centre was the scene of riots in March when protesters stormed the camp and helped several asylum seekers to escape.

The Baxter detention centre was built four months ago to house up to 1,200 people and take the pressure off camps such as Woomera.

Anger

Australia's stringent immigration policies have been criticised by human rights groups, with some asylum seekers detained for up to three years while their applications are dealt with by the government.

Refugee groups said that anger and boredom were the likely causes of the arson.

"They probably did it out of sheer frustration," said Marion Le from the Independent Council for Refugee Advocacy.

Western Australia's Project SafeCom Inc spokesman, Jack Smit, said of the Baxter fire: "I'm delighted that it's burning and I hope it burns right to the ground."

He said that a detention centre like Baxter "makes a mockery of the UN refugee convention... Australia has signed".