Police assisting in the interception of a suspected asylum seeker boat off WA's north-west coast have refused to comment on the boat's whereabouts or the fate of those on board after returning to port.

The boat, carrying what are believed to be Vietnamese asylum seekers, was spotted by crew members on an oil tanker at first light on Monday morning about 150 kilometres offshore.

Numbers could not be confirmed but those onboard appeared to be in good health, a spokesperson for oil and gas company Modec said yesterday.

Water police were called in to help locate the boat and the ABC understands an Australian Navy vessel followed later.

Police who returned to the Dampier port overnight would not comment on the operation to intercept the vessel, other than to say the boat was unlikely to dock at the port and that police would have no further involvement.

Prime Minister Tony Abbott has refused to provide any details in response to the reports, saying it is an "iron law" of his Government not to comment on operational matters.

"We do not discuss things in ways which would give aid and comfort to the people smugglers," he said.

"People will not come to this country illegally by boat. If any, by hook or by crook, actually get here they will never get permanent residency in this country."

WA Premier Colin Barnett said he believed naval vessels had taken over the handling of the boat from WA Police.

"The state police boat has been assisting the Commonwealth, I think their role is pretty well complete now and the Commonwealth naval vessels will take over," he said.

"But as we always do, the state always assists in these operations."

Children on board vessel: Refugee Action Coalition

The Refugee Action Coalition said the number of people on board was yet to be clarified, but it understood children were on board the boat.

"Can confirm there are children but no exact numbers or ages yet," spokesman Ian Rintoul said in a text to the ABC.

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Mr Rintoul said lawyers had been liaising with the Australian Vietnamese community to collect the names of some of those on board.

They are also investigating what action can be taken to assist the people on board, he said.

"I haven't had direct contact with the boat, but I do know that lawyers now have names of people on the boat and we are looking to see what legal action is possible to assist them," Mr Rintoul said.

He said it was clear the Government planned to return the boat back to Vietnam.

"I think it's very clear with the comments made from the WA police that the Australian Government intends to intercept them on the high seas, to hold them on the high seas and screen them out and return them to Vietnam," he said.

"That's exactly what happened with the boat of Vietnamese in April, and all of the signs are they will attempt to do exactly the same thing."

Christmas Island president Gordon Thomson confirmed the asylum seekers were not expected to be taken to the island.

"No preparations have been ordered here," he said.

"We expect the people will be taken to Darwin and flown back to Vietnam from there under the 72-hour turnaround policy."

Greens criticise level of secrecy surrounding situation

Yesterday, the Greens criticised the level of government secrecy surrounding the boat's arrival.

"There's no justification for the minister and the department to keep the Australian people and the Australian Parliament in the dark," Greens senator Sarah Hanson-Young said.

"Be upfront about what has occurred here, and just have the guts to tell the Australian people what indeed is going on at our borders."

Mr Rintoul echoed the Greens' concerns about the level of secrecy.

"The WA Police are not controlled by the Australian Government but when they returned this morning they refused to give information on the same kind of grounds, that there is some kind of on-water operational matter," he said.

"But there is clearly no issue of national security that's associated with asylum boats.

"The secrecy is simply designed to ensure there is no criticism, that there is no oversight, there is no awareness of what the Australian Government actually does with the asylum seekers.

"So it's really a self serving justification by the Australian Government to prevent any kind of public scrutiny or media scrutiny of how the Australian Government actually treats asylum seekers."

It is almost two years to the day since an asylum seeker vessel arrived at a floating oil platform 200 kilometres off Dampier with 17 people from Vietnam on board.