Conservation group Sea Shepherd claims it is under attack from the Japanese whaling fleet.

It says its ship the Bob Barker was hit by a harpoon vessel at 8.50am (New Zealand time) in the Southern Ocean, as the Japanese vessel crossed in front of its bow.

The incident is part of an ongoing attack by three harpoon vessels which started at 2am (NZT), Sea Shepherd says.

"During the attack, the harpoon vessels overtook the Sea Shepherd ships from the stern, crossing the bow, and coming as close as three to five metres," the anti-whaling group said.

"The whaling vessels have also made consecutive attempts to foul the propellers of the Sea Shepherd ships by dragging steel cable across the bow of the conservation ships."

Sea Shepherd says its crew had objects thrown at them and were sprayed with water cannons as they tried to cut the cables.

The group says its two ships, the Bob Barker and the Steve Irwin, have taken up a position blocking the loading ramp of one of the Japanese fleet, preventing the whalers from landing their catch.