Australia will shut four mainland immigration detention centres, the government said today, as it pushes ahead with controversial policies to turn back refugee boats and detain asylum seekers in remote centres offshore.

Immigration Minister Scott Morrison said in a statement the decision to shutter the facilities, which are run by British outsourcing company Serco Group Plc, would save at least A$88.8 million ($80.41 million) a year.

"These sites are remote, relatively small and expensive," Morrison's statement said. "These facilities were never envisaged as being permanent and due to the rationalisation of the immigration detention network they are no longer required."

Australia's conservative government routinely does not answer questions about its immigration policy, which was a central plank of its election victory last year, saying it does not comment on "operational matters".

The number of would-be refugees reaching Australia pales in comparison with other countries but it remains a polarising political issue that also stokes tensions with neighbour Indonesia over border policies criticised by the United Nations.

Today's announcement comes amid growing scrutiny of Prime Minister Tony Abbott's asylum seeker policies.

The government has refused to confirm widespread reports that the Australian Navy has started implementing a controversial policy of returning intercepted vessels carrying asylum seekers to Indonesia.

Asylum seekers hoping to reach Australia often transit through Indonesia, many paying people-smugglers thousands of dollars to make the perilous journey in unsafe boats.

Despite a shift towards processing asylum seeker claims in other countries such as Nauru and Papua New Guinea, there were more than 6,000 people in immigration detention facilities in Australia by Nov. 30, 2013, according to government figures.

Most of those held in offshore centres have fled conflict zones such as Afghanistan, Darfur, Pakistan, Somalia and Syria.

The mainland centres will be closed by the end of February, Morrison said, and any remaining detainees will be moved to one of another 16 facilities.

Morrison did not provide details on the numbers of detainees to be transferred but Australian media reported that there were about 250 people in two of the centres to be closed and that a third had been empty since September.

Richard di Natale, acting leader of the small but influential Greens Party, said he feared the government's policies would exacerbate an already overtaxed system.

"We've got a situation where all of our detention centres are under huge strain," he told the Sydney Morning Herald.

The Australian Broadcasting Corporation (ABC) reported that a hunger strike was underway at a Serco-run detention centre on Christmas Island, an Australian territory in the Indian Ocean closer to Indonesia than the mainland.

Several prisoners had sewn their lips shut as part of their protest, the ABC reported without citing sources. Human rights reports have for years chronicled incidents of self-harm, hunger strikes and riots in Australia's detention centres.

Serco is investigating the escape of three detainees from another detention centre at the weekend, the third breakout from one of its facilities in six months.

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