The Labour Party says Government action must be taken if the claims Japanese whalers are poaching in New Zealand waters are true.

Conservation spokesperson Ruth Dyson said the accusations by Sea Shepherd were concerning. The environment group claimed earlier today that Japan's whaling fleet had been found inside New Zealand's Ross Dependency region.

"The Government must verify them immediately. If the Japanese are whaling in our waters the Government must demand answers now," Dyson said.

"New Zealanders have a deep fondness for whales. The news that a pod of stranded whales has died on our beaches today is upsetting. To know that other whales in our waters are being slaughtered against our wishes will anger many Kiwis."

Sea Shepherd said earlier that it had located all five Japanese whaling vessels and filmed one with three dead minke whales on its deck.

The organisation's position data puts the whaling fleet about 1700 kilometres north of Scott Base, beneath the usual Christchurch to McMurdo Sound flight path.

Referring to its vessels Steve Irwin, Bob Barker and Sam Simon, Sea Shepherd said it was "in pursuit of the whaling fleet, driving them away from their intended poaching grounds, disrupting their illegal hunt, and preparing to shut down their whale-killing operations".

The factory ship Nisshin Maru was first located by a Sea Shepherd helicopter which filmed "compelling footage and images of three dead protected minke whales on the deck of the Nisshin Maru.

"A fourth whale, believed to be a minke, was being butchered on the bloodstained deck."

The area is part of the Southern Ocean Whale Sanctuary.

"The Southern Ocean Whale Sanctuary has been tainted by the illegal slaughter of these beautiful and majestic minke whales by the ruthless, violent and barbaric actions of the Japanese whale poachers," Sea Shepherd Australia managing director Jeff Hansen said.

"No-one will ever know the pain and suffering these playful, gentle giants went through from the time the explosive harpoon ripped through their bodies to the time they drew their last breath in a red sea full of their own blood."

New Zealand last year joined the Australian Government's challenge to the legality of Japan's whale hunt in the Southern Ocean Whale Sanctuary at the International Court of Justice. Judgment on the case is yet to be delivered.

The Southern Ocean Whale Sanctuary covers 50 million square kilometres of ocean from 40 degrees South to Antarctica. It is recognised by 23 countries.

Sea Shepherd said it located Nisshin Maru at 64 degrees 44 minutes south and 162 degrees 34 west. The Ross Sea covers an area between 160 degrees East and 150 degrees West and 60 degrees South. Although technically claimed as part of New Zealand's sovereign waters, the claim is suspended under the Antarctic Treaty.

The area the Japanese are sailing through is part of an area that the United States and New Zealand unsuccessfully tried to have declared as part of what would have been the world's largest marine protected area.

It was opposed by Russia, Ukraine and South Korea.

The area where the whalers and Sea Shepherd are likely to clash is within New Zealand's designated search and rescue area.