The boat is believed to have set off from Vietnam sometime before April.

It was only intercepted by the navy on Tuesday when it was within sight of Port Hedland, a mining town in Western Australia.

The boat's ability to evade both coastguard patrols and radar prompted the opposition Labor Party's immigration spokeswoman, Julia Gillard, to ask the government to explain its 'major failure in coastal surveillance'.

Last month, the navy said its surveillance capability had been reduced to just one Orion aircraft and one frigate as a result of the reduction in boat arrivals.

The Vietnamese, reportedly an extended family made up of men, women, seven children and a baby, were transferred to the frigate Canberra, which will take them to a detention centre on Christmas Island, an Australian territory south of Java. The 1,800km journey is expected to take three days. Although it would have been easier to take the refugees to a detention centre at Port Hedland, the government is anxious to prevent asylum-seekers setting foot on Australian soil, where they can then make a claim for refugee status.

Although Christmas Island is an Australian possession, it was removed from the legal definition of Australian jurisdiction as part of a tough package of laws introduced after August 2001, when a crisis developed over the rescue of 400 mostly Middle Eastern refugees by a Norwegian freighter, the Tampa.

Refugee advocate Marion Le told Australian Associated Press (AAP): 'It's a terrible waste of taxpayers' money in the sense that this boat will now be taken on a journey of about three days to get to Christmas Island in order to get them outside the migration zone, when they could be taken to Port Hedland.'

Ms Le said the involvement of the UN was needed to ensure the boatpeople were treated fairly.

'Australia has clearly shown itself to be outside the international law in the way in which they are dealing with and processing people. We need an independent arbiter.'

Allegations emerged yesterday that the refugees had been refuelled and resupplied by Indonesian authorities in West Java before continuing on their journey to Australia. Local police apparently gave the refugees help before the arrival of officials from the immigration department and the International Organisation for Migration.

'This is a lesson for us,' Daud Afifi, the head of immigration in the western Javanese city of Serang, told AAP.

'We briefed the police and the military so it won't happen again in the future,' he said.

Australia's Immigration Minister, Philip Ruddock, described the arrival as 'an isolated incident' and said Indonesian authorities had fully co-operated in trying to stem the flow of refugee boats to Australia. 'They've been very supportive of our regional efforts to combat people-smuggling,' he said.

He said the Vietnamese were probably travelling to Australia for economic rather than political reasons, and that it was not thought the arrival was organised by people-smugglers.

At least two other vessels carrying Vietnamese asylum-seekers have attempted to reach Australia in recent months, but both were forced to turn back to Indonesia when their vessels threatened to sink.

On Tuesday, Amnesty International directors from 14 countries - including Hong Kong, Australia, New Zealand, Japan, India and Thailand - condemned Australia's detention of 112 refugee children on the tiny South Pacific territory of Nauru as 'shameful'.