Australian Customs and immigration authorities are bracing for the single biggest influx of boat people in the nation's history.

Intelligence sources have reported that more than 1000 illegal immigrants are on their way through heavy seas from Indonesia, and are expected to arrive in Northern Australia today or during the weekend.

The news is a sharp blow to the campaign by the Immigration Minister, Mr Philip Ruddock, to halt the escalating people-smuggling rackets from the Middle East and China through Indonesia, and places intense pressure on Australia's overstretched detention facilities.

One vessel - either an inter-island ferry or a coastal freighter bound for Darwin - is believed to be crammed with as many as 700 boat people. It is facing high seas and monsoonal rain, sparking fears of a tragedy if it overturns or sinks.

Authorities, alerted by intelligence sources of the boat's departure from an undisclosed port in Indonesia, have told ships plying between Australia and Indonesia to report any sightings of the vessel.

Australian Coastwatch has another ship, carrying an estimated 350 people, under surveillance from the air as it limps towards Darwin.

A third boat, carrying about 70 people believed to be from the Middle East, is sailing to Ashmore Reef, off the far north Western Australian coast.

This follows the arrival on Ashmore Reef two days ago of 18 illegal immigrants who travelled via Indonesia from the Middle East.

Immigration officials, already stressed by the arrival of more than 2500 boat people since November, are frantically trying to plan how to house the latest flood.

Detention centres throughout Australia are already drastically overcrowded, despite the opening last year of a new centre at Woomera, in the South Australian outback.

The operation to track the illegal boatloads was being kept a closely guarded secret yesterday, apparently to protect Australia's intelligence network in Indonesia. Both the Foreign Affairs Department and Customs - which operates the Coastwatch surveillance network - are involved in the operation.

A spokesman for Coastwatch, Mr Leon Bedington, refused yesterday to confirm or deny the imminent arrival of the new wave of boat people.

"We receive intelligence from a wide range of sources," he said.

"We will not comment publicly on the validity or otherwise of any intelligence we may receive."

However, The Age learnt that the vessel carrying about 350 people and travelling towards Darwin had left Surabaya, on the Indonesian island of Java, within the past few days.

Intelligence services were keeping secret all details of the boat carrying 700 people.

The departure point for the smaller boat heading towards Ashmore Reef was Roti, a small island off the western tip of Timor.

Last month Mr Ruddock toured the Middle East and South-East Asia warning that illegal immigrants were not welcome in Australia.