Former Australian Greens leader Bob Brown says anti-whaling environment group Sea Shepherd has “huge” work to do following the ICJ’s ruling aginst Japan.

Former Australian Greens leader Bob Brown says anti-whaling group Sea Shepherd still has “huge” work to do in the wake of the UN’s top court banning Japan’s controversial hunt in the Southern Ocean.

Dr Brown stepped down as chairman of the militant environmental group on Tuesday after two years in the role, making way for former NSW Greens MP Cate Faehrmann.

His decision came after the International Court of Justice (ICJ) this week ruled to ban Japan’s “scientific” hunting program, which for years has seen Sea Shepherd and Japanese vessels face off in Antarctic waters.

Dr Brown said the ICJ decision didn’t diminish the “huge” role Sea Shepherd had to play in protecting the marine environment.

“It’s going to be growing,” Dr Brown told AAP on Thursday.

“We all know there’s whaling by Japan, Norway and Iceland in the northern hemisphere.”

Dr Brown said he expected the focus of Sea Shepherd’s Australia arm to shift from whaling to illicit fishing.

“The Antarctic fisheries are being marauded by the illegal fishers,” he said.

“The krill upon which the whale depends we think are going to be increasingly plundered to feed fish farms and pet food factories.

“There’s not much good saving the whales if they’ve got nothing to eat.”

Ms Faehrmann has already indicated she plans to focus on illegal fishing, particularly of sharks and krill, Western Australia’s and Queensland’s shark culling schemes and the preservation of the Patagonian toothfish.

“The oceans will need Sea Shepherd for many years to come, unfortunately,” she said on Wednesday.

Dr Brown said he felt it was “exactly the right moment” to finish up given the ICJ ruling.

“I expect to flow from that (ruling) the end of the extraordinary court action being taken against Paul Watson, the founder of Sea Shepherd in the United States,” he said.

“In a way, my task of being there when they took him out of action has been fulfilled.”

Japanese authorities had been seeking Mr Watson’s extradition, describing methods used by Sea Shepherd against whaling ships – for example blocking the boats’ propellers – as “terrorist” acts.

Dr Brown said he would spend the next few months with his partner touring bush heritage sites across the country.