The New Zealand Government is considering what pressure it can apply to Japan to get it to reverse a decision to resume whaling in the Southern Ocean.
The International Court of Justice ruled last year that Japan should stop its scientific research programme, and the country's fisheries agency subsequently cancelled most of its whaling for the 2014-15 season.
Japan has now notified the International Whaling Commission of its new plan for whaling this season, which reduces the annual catch of minke whales by two thirds to 333.
Michael Lawry, the director of conservation group Sea Shepherd, said if Japan went ahead with whaling, it would be in breach of at least three international laws.
"They can't be told any more firmly than they've already been told by the rest of the world and the highest court in the world, so something has to be done.
"I think the next level is sanctions or our government physically stopping them in the ocean."
Sea Shepherd was unlikely to have the resources to tail the Japanese whaling ships this season as it has done in the past, Mr Lawry said.
New Zealand's acting Foreign Minister Todd McClay said as well as the court ruling, Japan was defying a recommendation made earlier this year by the IWC's expert panel, asking the country to undertake further work on whether non-lethal methods could achieve its research aims.
"It's deeply disappointing. New Zealand stands strongly against any whaling in the Southern Ocean and we view this as a backward step."
Mr McClay would like to see Japan postpone its plan for this season, to give the IWC the chance to consider its proposal next year.
"We'll need to continue to advocate strongly against whaling in that forum but until we see exactly the justification that Japan is using to recommence whaling, we'll just have to wait and consider our options."
Japan began its whaling programme in 1987, a year after an international moratorium was introduced.