The governments of New Zealand and Australia are being accused of doing nothing to stop the slaughter of 333 minke whales in Antarctica.
Japanese whalers have just returned home after killing the mammals, including 230 pregnant females, one of whom had twin calves.
It's the first time Japanese whalers have returned to the Southern Ocean since their whaling programme was ruled illegal in 2014.
Japan's Institute of Cetacean Research -- the private, government-funded body that conducts Japan's whale hunts -- yesterday disclosed whaling coordinates that showed that many of the whales were slaughtered in the Southern Ocean Whale Sanctuary and Australian Whale Sanctuary.
Sea Shepherd New Zealand director Michael Lawry said international powers were not doing anything to stop Japan because it was a powerful trading partner.
He said if another country's fishing vessel was illegally taking wildlife, then the international community would go after them.
Mr Lawry said international communities should be upholding international law, not sitting back and watching it happen.
The Sea Shepherd group was getting a fast, long-range vessel, which would help in the fight against whaling.
However, the group said the whale sanctuary should be patrolled by governments on both sides of the Tasman.
Sea Shepherd founder advisor Paul Watson strongly condemned Japan's latest violation of international law.
"This is the same pattern we've seen for years: Japan simply ignores international law and international opinion, and continues to slaughter whales with impunity, selling their flesh for a profit," he said.
"Nearly two years have passed since the International Court of Justice ruled Japan's commercial whaling program illegal, and yet the whalers are still announcing hundreds of fresh kills, including of pregnant mothers. The world must unite to end this lawless bloodshed once and for all."
The International Court of Justice in 2014 ruled that Japan's Antarctic whaling program was not scientific as Tokyo had claimed and must stop.
Japan last year conducted only non-lethal research into whaling, but it says killing whales is essential to obtain data on their maturing ages.
Scientific research is exempt from a 1986 international ban on commercial whaling.
Opponents of Japan's Antarctic hunt say it's a cover for commercial whaling, since the surplus is sold.
The catch quota under the new research program is about one-third of what Japan used to kill.
Its actual catch has fallen in recent years in part because of declining domestic demand for whale meat. The government has spent large amounts of tax money to sustain the whaling operations.