The anti-whaling ship the Steve Irwin has clashed with the Japanese in the Southern Ocean for the first time this whaling season.
Captain Paul Watson said the Japanese security ship the Shonan Maru 2, which has been tailing them since December 9, opened fire with two water cannons in international waters at 2pm on Monday.
The Steve Irwin crew, wet but reportedly in good spirits after the confrontation, returned fire.
The incident happened when the Australian vessel looped around an iceberg, did a figure-eight out of view of the Japanese vessel, and re-emerged within a few hundred metres of the whaler.
The Steve Irwin, intent on shaking off the Shonan Maru 2, is heading for icy waters, hoping to outmanoeuvre the Japanese ship.
Sea Shepherd leader Paul Watson told the Sydney Daily Telegraph he believed that military personnel had boarded the vessel, by the way the crew acted and operated in military style.
"The way they operate [and] the way they move about the ship is very different to past years," he said.
"It has four sonic cannon this year - last year they had one - and there are spikes all along its side and netting to stop boarding."
Mr Watson worked for Greenpeace before founding the Sea Shepherd Conservation Society.
The Shonan Maru 2 has been deployed to track the Steve Irwin to try to keep it away from the whaling fleet. A Japanese spotter aircraft has also been keeping constant watch.
This season Japan had permission to kill 935 minke whales, 20 fin whales and 50 humpbacks, Mr Watson said.
Japanese Prime Minister Yukio Hatoyama has told Australian Prime Minister Kevin Rudd that the Sea Shepherd's actions threatened the safety of the whaling fleet and asked that his Government takes immediate action against the vessel.
But Mr Rudd threatened legal action against the Japanese over the hunting of whales if "a diplomatic resolution proves difficult".