Environmental campaigners from the militant Sea Shepherd group said Wednesday a Japanese whaling ship had "rammed" its vessels in a skirmish in the Southern Ocean.
Sea Shepherd director Bob Brown said the Japanese factory boat the Nisshin Maru had deliberately collided with boats crewed by anti-whaling campaigners, the Steve Irwin and the Bob Barker.
Brown, the former leader of Australia's Greens party, said the Japanese boat had "repeatedly rammed" the Australian boats, adding that the Japanese had also lobbed concussion grenades onto their decks.
"This is grand piracy," he said, adding that the actions of the Japanese breached a number of international and domestic laws.
Sea Shepherd spokesman Jeff Hansen said the Steve Irwin had been struck on the stern and right hull.
"We're now under attack with concussion grenades," he told ABC television.
"We're very concerned for the safety of our crew at the moment."
The Sea Shepherd Conservation Society is chasing the Japanese fleet hunting whales off Antarctica in a bid to harass the whalers and prevent the slaughter of animals.
Environment Minister Tony Burke has described Japan's whale hunt as cruel and unnecessary but has so far rejected calls to send an Australian government vessel to monitor the hunt.
Australia strongly opposes whaling and launched legal action challenging the basis of Japan's so-called "scientific" hunt in December 2010.
Japan claims it conducts vital scientific research using a loophole in an international ban on whaling, but makes no secret of the fact that the animals ultimately end up on dinner plates.