A former defence force chief has warned the Coalition's policy to turn back asylum seeker boats could amount to an act of piracy.

The Opposition says one of its options to "stop the boats" will be to turn back vessels in international waters where it is safe to do so.

But Admiral Chris Barrie says if Australian authorities try to board and redirect a boat found in international waters, that could break international piracy law.

"The question will be, is that vessel in some sort of peril or is it not?" he said.

"If the boat itself is well-found and it appears to be exercising freedoms of the high seas, a right to board is very much constrained under international law, and there would be some circumstances, I guess, where it might become an act of piracy.

"I think the authorities here would want to make sure that they understood precisely what was going on, but I think the fundamental issue under the law of the sea agreement is on the high seas, everybody exercises the right of freedom of navigation."

But Associate Professor Tim Stephens, an international law professor from the University of Sydney, says he does not think the Coalition would face piracy charges, but it could run into other problems.

"Were Australia to interfere with the vessels of other countries, then Australia almost certainly would be subject to international condemnation and could well be subject to proceedings in the international tribunal for the laws at sea," he said.

Admiral Barrie was defence chief during the children overboard affair and the 2001 Siev X disaster under the Howard government.

At the time he initially backed - and then rejected - claims that asylum seekers had been throwing their children overboard.

His latest remarks come two days after a boat carrying 66 Sri Lankan asylum seekers managed to sail into Geraldton in Western Australia.

The Federal Government was outraged by suggestions from Opposition Leader Tony Abbott that it had "surrendered" in the battle to curb boat arrivals.

The Opposition says the Geraldton boat arrival has bolstered its policy for unmanned drones to patrol Australia's waters. It wants to buy at least seven of them at a minimum cost of $1.5 billion.

The arrival on Tuesday was the first case in five years that an asylum seeker boat had reached the mainland.

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Admiral Barrie says Australian authorities would struggle to find the resources to escort any boat back to its country of origin.

"The bottom line would be... if this boat is from Sri Lanka and we were now obligated to take this boat back to Sri Lanka under some sort of escort arrangement, well, that's quite a long way to go," he said.

"I'm not sure that we would have resources to do more than something like that every now and then.

"The current proposal in front the Parliament at the moment, I think it's got all sorts of implications for the way we manage people arriving in Australia and wanting asylum and our obligations under the refugee conventions."

The Immigration Department said all 66 asylum seekers would be processed at Christmas Island.

Greens MPs are calling on the Government to guarantee the asylum seekers will have access to lawyers.