Australia has vowed to stop asylum seekers reaching its shores, turning boats back to Indonesia when it can and sending those it cannot to camps in impoverished Papua New Guinea and Nauru in the South Pacific for long-term detention.
Australia's Department of Immigration routinely does not comment on "on water" operations, so it was not immediately possibly to verify the ABC report.
The ABC cited a source within the federal government confirming the sighting of the boat near Dampier, about 1,500 km (930 miles) north of Perth. A police boat was being sent to intercept the vessel, it reported.
The last asylum-seeker vessel to come so close to the Australian mainland was a boat carrying 17 people from Vietnam, which arrived in late July 2013 at a floating oil platform located about 200 km (125 miles) from Dampier.
Those asylum seekers were the last to be taken to the Australian territory of Christmas Island for processing before former Labor prime minister Kevin Rudd declared a new policy of offshore processing.
Asylum seekers have long been a lightning-rod political issue in Australia, although it has never received anywhere near the number of refugees currently flooding into Europe in an effort to flee growing instability in North Africa.
No one processed for asylum in Nauru or Papua New Guinea is eligible to be settled in Australia, even if they are found to be a genuine refugee, under the harsh policies introduced by Rudd and strengthened by the current conservative government.
The United Nations and human rights groups have criticised Australia over its tough asylum-seeker policies. Conservative Prime Minister Tony Abbott has defended the measures as necessary to stop deaths at sea.
In June, reports that Australia paid people-smugglers bound for Australia thousands of dollars to turn their boat back to Indonesia caused tensions with Jakarta, plunging relations to their lowest point in more than a year.
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