Environmental activist group Sea Shepherd says it is having its most successful season yet in its fight against Japanese whalers in the Southern Ocean.

The group's Peter Hammarstedt has told Radio National the Japanese fleet has only managed to kill between 30 and 100 whales so far, a fraction of their quota of 1,035.

He says Sea Shepherd is confident the fleet will only be able to catch under 10 per cent of its quota this season.

"This year we had a very specific goal that we were going to try to get the Japanese whaling quota down as close to zero kills as possible," he said.

"And this year we'll be surprised if they even get 10 per cent of their quota. We've been with these guys from the first day of the hunting season."

Mr Hammarstedt, first officer on the Sea Shepherd ship Bob Barker, says the strategy of placing vessels in the slipway of the processing ship Nisshin Maru Two has severely hampered the whaling outfit.

"Our strategy is pretty simple, we deliberately place our ship right on the slipway of their processing ship," he said.

"The whaling fleet consists of a floating slaughterhouse and three harpoon ships.

"And if we're able to manoeuvre our vessel in a way that the harpoon ships can't get close enough to the processing vessel, then we completely block all their operations.

"What we've found is that our 500 tonnes of steel sitting right on their stern has put us in a very powerful negotiating position with the whaling fleet."

Financial squeeze

He says the disruptions are costing the fleet millions, and Sea Shepherd is hopeful this financial pressure will bring about the end of Japanese whaling.

"The whaling fleet has been desperately trying to get away from us, every day we're with them, we're preventing them from killing about 10 to 20 whales.

"We speak the only language these poachers understand, that's the language of profit and loss. And right now we're costing the industry, that's already $200 million in debt to the Japanese government, even more in lost profits.

"And so I think we're really approaching the inevitable and this year or the year after we should see an end to Japan's so-called research down here.

"I think we've got the majority of public opinion on our side, we've got the law on our side in this matter, and it's just a question of time before we see their illegal whaling operations cease completely down here."