SYDNEY (AFP) - A fishing boat packed with 66 asylum-seekers evaded detection in a "shocking" breach of border security to arrive at a busy port on the Australian mainland on Tuesday, officials said.

The vessel was spotted within the harbour limits of Geraldton in Western Australia, more than 2,000km south of Australia's Indian Ocean territory of Christmas Island where asylum-seekers are usually intercepted.

"Customs and Border Protection have advised a suspected irregular entry vessel arrived within the harbour limits of Geraldton this afternoon," Home Affairs Jason Clare confirmed.

"Initial indications suggest there are 66 people on board."

The West Australian newspaper said the boat, with men, women and children on board, was from Sri Lanka and had been at sea for more than six weeks before arriving at the port 425km north of Perth.

Western Australia Premier Colin Barnett said he was "alarmed" that a boat could make it undetected to Geraldton, one of Australia's busiest regional ports and the country's second-largest for grain exports.

"This is a serious, unprecedented and unacceptable breach of Australia's border security," Mr Barnett told reporters.

"That a boat, laden with people, can sail into a busy regional port in broad daylight is shocking."

Australia is facing a steady influx of asylum-seekers arriving by boat, many of whom use Indonesia as a transit hub. They pay people-smugglers for passage on leaky wooden vessels after fleeing their home countries.

Thousands also come from Sri Lanka, with Canberra currently funding a media campaign in the South Asian country to caution would-be boatpeople against undertaking the journey.

It is an inflammatory political issue in Australia and certain to dominate national elections due in September, despite the fact that overall arrival numbers are relatively low by global standards.

Mr Steve Ranch, who manages the local Dome cafe in Geraldton, told reporters that locals were stunned to see the wooden fishing boat approaching the shore about noon.

"At first people weren't sure what was going on," he said.

"We thought it was a hoax or a publicity stunt, but then we saw the customs towing it away."