A senior Customs official has publicly questioned the Opposition's strategy of using drones to help detect illegal vessels entering Australian waters.

The Coalition wants to purchase seven of the high-tech Triton unmanned aircraft.

Earlier this year Opposition defence spokesman David Johnston declared the drones would form an important part of the party's much repeated pledge to "stop the boats".

But those who would be charged with making it work have cast doubt on the Triton's ability to fulfil such precise policy outcomes.

Nic Arthur, the manager of maritime capability with the Customs and Border Protection service, says the drone may not be able to see the small wooden vessels most often used by asylum seekers.

"An additional seven aircraft would be terrific," he told a conference in Canberra on Wednesday.

"[But] is a high altitude long endurance UAV the best tool for providing detection of small wooden vessels? I think the jury is still out.

"If flying at a great altitude was going to provide us greater coverage we'd be doing that with the Dash 8 aircraft now."

When questioned by the ABC in Perth later in the day, Opposition immigration spokesman Scott Morrison walked the previous pledge back just a little.

"There's more than just that drone technology and the Coalition has made no commitment," he said.

"Aerial surveillance is critical not just for the point of view in conducting interceptions but being able to act proactively in terms of search and rescue as well."

The Government has not ruled out buying the Triton either, although so far it is still investigating the cost and capability.