Sea Shepherd claims to have found a Japanese ship in the Australian Whale Sanctuary with a dead minke whale on its flensing deck.
The Nisshin Maru whaler factory ship was spotted by the helicopter of Sea Shepherd's MY Steve Irwin as the crew allegedly scrambled to hide the slaughtered whale with a tarp, while the fleet's harpoons were also quickly covered.
"The whale killers from the Nisshin Maru were caught red-handed slaughtering whales in the Australian Whale Sanctuary," Ocean Warrior Captain Adam Meyerson said on Sunday.
"The Steve Irwin has shut down their illegal operations and caught them trying to hide the evidence."
It is the first documented kill since the International Court of Justice ruled against Japan's whaling operations in the Antarctic in 2014.
It comes after Japanese Prime Minister Shinzo Abe visited Australia on an official state visit.
Jeff Hansen, Managing Director of Sea Shepherd Australia said, "The lack of action by the Turnbull government while whales are being killed in Australian waters just a day after Japan's Prime Minister was on a state visit in Australia shows that the government has no spine when it comes to protecting the wishes of Australians to defend the Southern Ocean Whale Sanctuary."