Paul Watson, the captain of the second ship Farley Mowat, told local radio that the New Zealand Government knew the location of the Japanese whalers because its air force had filmed the fleet.
The Sea Shepherd ships have approximately three weeks before they must leave the area to refuel and pick up supplies.
On the Sea Shepherd website, Watson said he believed the Japanese fleet was within 850km of his ships.
International environmental group Greenpeace set sail from New Zealand on Friday to start its 2007 anti-whaling campaign, trying to come between Japanese whalers and their prey in the Southern Ocean.
A global moratorium on commercial whaling has existed since the 1980s, but Japan kills hundreds of whales each year under a scientific research permit, which opponents say is used to whale commercially in disguise, often with the whale meat ending up in high-end restaurants.
Iceland and Norway are the only countries to ignore the moratorium and conduct commercial hunts.
Japan has called a special February meeting of members of the International Whaling Commission in an attempt to help lift the whaling moratorium, but 26 anti-whaling nations, including Australia, have said they will boycott the meeting.
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