Anti-whaling activists from the Sea Shepherd Conservation Society have admitted to bugging Japan's whaling fleet with high-tech tracking devices in order to follow them.

Sea Shepherd's vessel the Steve Irwin caught up with the Japanese whaling fleet near the Shackleton Ice Shelf on Saturday, having returned to Antarctic waters from a refueling pit stop in Melbourne.

Sea Shepherd leader Paul Watson said the bugs had been planted on the Japanese harpoon boat Yushin Maru 2 when two of its crew boarded the vessel last month.

"That's why when we left Melbourne it only took us nine days to find them. We went straight towards them," Mr Watson said.

"I think it's really funny. They are probably ripping the ship apart right now trying to find them, but they aren't going to find them."

Mr Watson said further tracking devices had also been planted on some of the other Japanese vessels.

"We are not saying which ones, or how we did it, but we have the ability to follow the fleet and that's what we are doing right now," Mr Watson said.

A spokesman for the Institute of Cetacean Research, which conducts Japan's scientific whaling program on behalf of the Japanese government, admitted it was possible the Yushin Maru had been bugged.

"Given that the Sea Shepherd found the fleet so quickly it is certainly one explanation," spokesman Glenn Inwood said.

He said it was also possible the two Sea Shepherd activists, who boarded the Yushin Maru on January 15, were treated too lightly by Japan.

The incident sparked a three-day standoff before the Australian customs vessel the Oceanic Viking intervened.

Mr Inwood said he did not believe the other Japanese ships had been bugged.

"It's seems very unlikely. There hasn't been any opportunity for the Sea Shepherd to have planted devices on any of the other vessels," he said.

Mr Watson said the Sea Shepherd had managed to chase the Japanese fleet for the past three days, during which no whales had been killed.

"We will keep this up until the middle of March, by which time the weather will have turned and they (the whalers) will have no choice but to give it up," he said.

Mr Watson estimated the whalers had managed to kill no more than 400 whales since the season began in November, which would make it almost impossible for Japan to reach its annual quota of up to 935 minke whales and 50 fin whales.

- Kyodo