Crew on board Sea Shepherd ship Steve Irwin say they have located two more Japanese ships in as many days in the Southern Ocean.
The conservation group hopes to thwart the stated plan of the Japanese fleet to hunt and kill 333 Antarctic minke whales this summer.
The captain of the Steve Irwin, Wyanda Lublink, said they had now spotted four vessels altogether - two harpoon ships, a research ship and a refuelling tanker.
Ms Lublink said she and her fellow activists were confident they would find the main Japanese whaling ship, the Nisshin Maru, very soon.
The two harpoon ships both had their harpoons uncovered, which was a sure sign that what she called the "slaughterhouse ship" was not far away, she said.
"We're pretty sure that the Nisshin Maru is nearby, 'cause otherwise there's no reason for them to have their harpoon uncovered. It's always been an indication in the past that the Nisshin Maru is in the area."
Ms Lublink said her ship and its sister ship Ocean Warrior were determined to stop the Japanese fleet.
New Zealand participated in the International Court of Justice case that ruled Japan's whaling programme illegal in 2014, and has continued to state its opposition to it.
Japan has been using a scientific loophole to continue hunting the animals since a moratorium on whaling was passed in 1996.
It argues its annual hunt is necessary to study whale numbers and make observations about them.