Fifteen people have been rescued after a boat carrying about 40 people sank off Western Australia, an official said on Monday, adding there were "grave concerns" for the others.

A merchant ship and Taiwanese fishing vessel are searching for more survivors in rough, pre-dawn seas after the boat got into trouble about 700 nautical miles northwest of the Cocos Islands.

"We obviously have grave concerns about the safety of those who are still in the water, given they've been in there for some time now," a spokeswoman for the Australian Maritime Safety Authority said.

"The LNG Pioneer (merchant ship) is still conducting search and rescue now," she added. "It's throwing out its life-raft in the hope of recovering more people from the water."

Authorities have not confirmed whether the sunken boat was one of dozens of people-smuggling vessels which have headed to Australia this year carrying asylum-seekers, many of them from Sri Lanka and Afghanistan.

The boat got into trouble overnight on Sunday, prompting a plea for help by Australian authorities which was answered by the LNG tanker and Taiwanese fishing craft.

"We received a distress call from the vessel yesterday morning when they indicated they were taking on water," the maritime safety spokeswoman said.

"We then issued a broadcast to shipping and that's how the two vessels that are there arrived on the scene."