Foreign Affairs minister Murray McCully says New Zealand will not ban anti-whaling protest group Sea Shepherd Conservation Society, despite a request by Japan.
Japan has blamed continued harassment by the group for its decision to halt this year's whale hunt in the Southern Ocean early.
Japan's whaling organisation the Institute of Cetacean Research says New Zealand is obliged to act because Sea Shepherd use its ports to refuel, although none of the protest organisation's vessels are New Zealand-flagged.
Institute spokesperson Glenn Inwood says Sea Shepherd is a vigilante terrorist group committing acts of violence on the high seas, and New Zealand has responsibilities under international law to make sure its ports are not used to harbour those who commit criminal acts.
After Japan called off its whale hunt, the Japanese Government called in ambassadors from New Zealand, Australia and the Netherlands, and asked them to take action against Sea Shepherd.
But Mr McCully says New Zealand operates a policy of open ports and there would need to be a very good reason to deny access. Currently, he says, there is no reason to do so.
He says the Southern Ocean is in New Zealand's search and rescue zone but this country has no jurisdiction in the area.
"We can't play policeman down there, we have no legal authority to take any steps against anyone."
Mr McCully says various conventions give responsibility to the flag nations and no New Zealand flagged ships were involved this year.
"Any actions that are going to be taken need to be taken by the flag nations of the vessels affected."
'900' whales saved
Sea Shepherd founder Paul Watson says the protesters won't stop until Japan stops hunting whales for good.
He says the whaling fleet is heading home after taking less than a tenth of its quota this season, which means 900 whales have been saved.
Mr Watson says Sea Shepherd plans to escort the fleet for a thousand miles to make sure the ships are well and truly out of the Southern Ocean.
While he hopes the cancellation is a permanent measure the battle is not over, he says, and if the ships return next year the protesters will too.