Two vessels from the environmental organisation left Australia on December 5 bound for the freezing Southern Ocean in the Antarctic for its 11th campaign to disrupt the Japanese hunt.
It said Ocean Warrior, built with financial support from the Dutch, British and Swedish lotteries, had already found one of the harpoon ships which meant the rest of the fleet would be near by.
"The crews of the Ocean Warrior and the Steve Irwin have been battling through thick fog and ice to protect the whales in the Australian whale sanctuary," said Ocean Warrior captain Adam Meyerson, referring to the sanctuary around Australia's Antarctic territory.
"Finding one of the hunter killer ships hiding behind an iceberg in a thick fog means that the rest of the fleet is nearby."
He added that the group hoped to "have whaling in the Southern Ocean shut down by Christmas".
The Ocean Warrior has a powerful water cannon and is capable of outrunning the whalers.
Japan has previously sought court action to halt the anti-whaling campaigns, saying the activists ram their ships, snare propellers with ropes and harass crew with paint and stink bombs.
The Japanese fleet set sail on November 18 in defiance of a worldwide moratorium on commercial whaling and international opposition.
Japan is a signatory to the International Whaling Commission's moratorium in force since 1986. But it exploits a loophole allowing for whales to be killed for the purposes of scientific research.
Tokyo claims it is trying to prove the whale population is large enough to sustain a return to commercial hunting for a traditional source of food. But the meat from what it calls scientific research often ends up on dinner tables.
In 2014 the United Nations' International Court of Justice ordered Tokyo to end the Antarctic hunt, saying it found permits issued by Japan were "not for purposes of scientific research".
After the ruling Japan cancelled its 2014-15 hunt, only to resume it the following year under a new programme with a two-thirds cut in the target catch number, saying the fresh plan was genuinely scientific.