A Japanese ship involved in security for Japan's whaling fleet has crossed into New Zealand's exclusive economic zone in the Southern Ocean.
Foreign Affairs Minister Murray McCully said the Shonan Maru No. 2 crossed early on Friday into the zone, which extends 200 nautical miles from the coast.
Mr McCully said the vessel was asked not to enter the waters, and although he's disappointed it did, it had a right to do so.
He said the Japanese government is well aware of this country's opposition to whaling in the Southern Ocean.
The Green Party said the Government should lodge a diplomatic protest after the ship entered New Zealand's exclusive economic zone.
Green Party oceans spokesperson Gareth Hughes said Mr McCully should protest through the New Zealand embassy in Tokyo.
He said New Zealanders find it deeply offensive to have Japanese whalers within New Zealand waters and the Government should act on their behalf.
Sea Shepherd said the security ship Shonan Maru No. 2 crossed while tailing the Sea Shepherd vessel Steve Irwin, and it should have been ordered out of the zone through diplomatic pressure. The Japanese vessel was involved in the sinking of protest boat Ady Gill in 2010.
Japan's whalers say they ignored the New Zealand Government request not to cross into its exclusive economic zone so they could defend themselves.
A spokesperson for Japan's Institute of Cetacean Research, Glenn Inwood, said every vessel has a right to be in the waters and New Zealanders would understand Japan needs to defend itself from Sea Shepherd.
Mr Inwood said Sea Shepherd's free access to New Zealand ports allows the group to commit acts of violence against Japan on the high seas after stocking up with fuel and supplies.
The Shonan Maru No. 2 did not cross into New Zealand's territorial waters - the area within 12 nautical miles of the coast - and Mr McCully said that was not expected to occur.