Australian Prime Minister John Howard, who has enforced a tough border protection policy, said the detention ship was appropriate as Australia faced a stream of illegal fishermen.

''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 on 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.