"The need for it is obvious. You can have a situation where you could have a surge of illegal fishermen and women and a holding capacity of that kind is very sensible," he said.

"This is a particular need in a special situation," Howard told reporters Thursday.

Civil libertarians, the Labor opposition and the Australian Greens party all condemned the prison ship plan as uncivilised, drawing comparisons with British convicts sent from overflowing prison hulks to Australia more than 200 years ago.

"A couple of centuries ago people were condemned to hulks for stealing bread," said Pauline Spencer, from Victoria state's Federation of Community Legal Centres.

"Now we're looking at people being condemned to these hulks for stealing fish...," she told reporters.

Australia, which automatically detains asylum seekers arriving by boat on remote islands or outback centres, has intercepted 234 foreign fishing vessels this year, more than double the number caught last year.

Most of those detained are Indonesian fishermen seeking lucrative shark fins. They are fined and sent back to their villages but their ships are burned.

Australian Customs has advertised for tenders to lease a vessel that will act as an offshore detention ship capable of holding 30 detainees and be fitted with deck-mounted machine guns, a spokesman for Fisheries Minister Eric Abetz said.

The ship would be used to secure illegal arrivals at sea for two to three days, allowing navy vessels to keep patrolling

instead of immediately returning to port with detainees.

"We need a vessel that will be able to hold illegal fishers out at sea so we can round up even more of the boats," Abetz told reporters.

Australia already operates a detention-supply ship, the Oceanic Viking, in its cold southern waters. It is used to hold illegal fishermen caught with valuable Patagonian Toothfish.

In March, the Oceanic Viking was deployed for two weeks off northern Australia where it held two groups of 40 Asian fishermen, ferrying them to the tropical port of Darwin.

Its success prompted the decision to deploy a detention ship in northern waters.

Australia will spend A$10 million ($7.6 million) a year leasing the new ship, which could be operating by early 2007.

"This policy harks back to Australia's penal history, when we should be at the forefront of compassionate and human solutions to border issues," Greens Senator Kerry Nettle said in a statement.

The Australian Council for Civil Liberties said the plan should be ditched. It was concerned about human rights abuses because the detention ship would keep detainees far from public scrutiny.

In May, Australia said it would spend A$500 million toughening security of its remote northern border with more surveillance flights and sea patrols. Nearly A$400 million will be spent combating illegal fishing.