Customs officers and West Australian police are on their way to intercept an asylum seeker boat off the Pilbara coast.
The ABC has been told a 20-metre vessel arrived at a floating oil platform 200km off Dampier on Friday morning.
Customs says it is unclear how many people are on the boat.
It is the third time in recent months a vessel has arrived near the coast undetected.
The ABC reports a boat carrying asylum seekers from Sri Lanka it turned up at the Port of Geraldton, 400km north of Perth, in April.
On Sunday, a boat was picked up 5km from the Broome shore.