Militant anti-whaling activists Sea Shepherd said they had zeroed in on a Japanese fleet Monday and captured evidence that four whales had been slaughtered, alleging the ships were found inside a Southern Ocean sanctuary.
Sea Shepherd said it had located all five Japanese vessels and was now in pursuit, forcing the harpooners to cut short their operation and retreat.
The group released footage and photographs showing three minke whales dead on the deck of the factory ship Nisshin Maru and said a fourth, also believed to be a minke, was being slaughtered when Sea Shepherd's helicopter flew overhead.
"There's three carcasses on the ship, a fourth carcass has been cut up. There's blood all over the place, meat being carted around on this factory ship deck, offal and innards being dumped in the ocean," said Sea Shepherd Australia chairman Bob Brown.
"That's just a gruesome, bloody, medieval scene which has no place in this modern world."
When the Nisshin Maru was first spotted from the air Brown said it was in Antarctica's Ross Dependency, within New Zealand's territorial waters and the Southern Ocean Whale Sanctuary, which he described as a "gross breach of international law".
The commercial hunting of whales is prohibited in the sanctuary, which was designated by the International Whaling Commission in 1994, but Japan catches the animals there under a "scientific research" loophole in the moratorium on whaling.
Brown described "massive violence" against the whales, using grenade-tipped harpoons to catch them, and said Sea Shepherd would do "all it peaceably can to prevent this grotesque and cruel destruction".
"I call on (Australian Prime Minister) Tony Abbott to have the spine to stand up to Tokyo and defend these whales," he added.
"The one thing that's missing here is gumption -- a bit of spine in Canberra and in Wellington to put an end to it."
Australia has taken Japan to the International Court of Justice seeking to have its research whaling programme declared illegal, with a ruling due this year.
Peter Hammarstedt, captain of the Sea Shepherd ship Bob Barker, said Japan had shown "flagrant disregard for international law by continuing their illegal whale hunt while the world patiently awaits a decision from the International Court of Justice".
Sea Shepherd left from Australia for their 10th annual harrassment campaign of the Japanese fleet last month, sending three ships to tail and run interference against the harpooners.
High-seas clashes between the two groups are common, resulting in the 2010 sinking of the Sea Shepherd vessel Ady Gil.
Australia will be monitoring confrontations between the pair from a government jet which is due to fly surveillance missions over the Southern Ocean between January and March.