Militant anti-whaling group Sea Shepherd on Monday dismissed as a "sham" Australian government plans to track Japan's annual whale hunt by air instead of by sea, saying it was a toothless and "cowardly" response.

Australia on Sunday announced an aerial Customs and Border Protection mission to the Southern Ocean as a showdown looms between Japan's whaling fleet and Sea Shepherd activists, saying it would send a message to both sides that the world was watching.

Environment Minister Greg Hunt had first promised to send a government ship to tail the warring groups but said the Airbus A319 would increase the surveillance mission's reach and effectiveness.

Sea Shepherd criticised the move, with Australian chairman Bob Brown describing it as a "pretty, cowardly" backdown by the conservative government in a bid to appease Japan due to ongoing free trade negotiations.

"They'll fly over and look from a great height. What are they going to do if something's going wrong down there? Where are they going to send a vessel from? Because those planes are not going to be able to intervene," Brown told reporters.

"It's a sham operation, this plane."

Instead of patrolling the southern fisheries, Brown said the government's custom-built Antarctic vessel ACV Ocean Protector was on border patrol duties between Australia and Indonesia as part of a military-led crackdown on people-smuggling.

"It is dangerous down there, we've got a violent Japanese whaling fleet on the way with grenade-tipped harpoons and it is dangerous to get in the way of that and uphold the law," he said.

Brown said Sea Shepherd's fleet had "three good days sailing" since departing Australia last week and they would beat the Japanese harpooners to the Southern Ocean.

"We'll be there when they arrive," he said.

"Now Greg Hunt has said there will be a plane overhead sometime in January and February -- the whaling fleet's going to be there before new year and we warned the government of this.

"This surveillance operation should be there from the outset, (instead) it's going to fail from the outset."

High-seas clashes between the Japanese and Sea Shepherd are common, with the activists regularly pelting the whaling ships with stink bombs, attempting to foul propellers and manoeuvring their vessels between harpoons and whales.

Both sides claim to have been rammed by the other over the years and Sea Shepherd's vessel Ady Gil sank during confrontations in January 2010.

Japan hunts under a "scientific research" loophole in the international moratorium on whaling.

Australia says this is illegal and has taken its key Asian trading partner to the International Court of Justice seeking an injunction against the harpoon programme. A ruling is due next year.