AFP - Australia said it was "deeply disappointed" after a fleet of Japanese whaling ships set out to kill hundreds of the giant ocean mammals on their annual hunt.

Environment Minister Peter Garrett urged Japan, which says the killings are for research purposes, to "swap harpoons for science" and study whales by other means.

"We are deeply disappointed that the Japanese Government has again embarked on its annual hunt to the Southern Ocean," Garrett said in a statement late on Thursday.

"The Australian government has said repeatedly that we do not have to kill whales to study them."

Greenpeace said four whaling ships left Japan for a five-month hunt in the Southern Ocean, using a loophole in an international moratorium that allows their killing for lethal "research".

Anti-whaling nations led by Australia and New Zealand and environmental groups including Greenpeace have long attacked the expeditions as cruel and unnecessary.

Militant activists have disrupted previous hunts by pursuing the whalers, leading to a series of angry confrontations involving rancid butter, stink bombs and, allegedly, ear-piercing sonic weapons.

The environmentalists have vowed to harass the ships in coming months using a futuristic, super-fast powerboat that last year clocked the fastest circumnavigation of the world on record.