Sea Shepherd conservation society spokesman Paul Watson told ABC Radio today that the $1.5 million vessel, the Ady Gil, which collided on Wednesday with a harpoon vessel, sank when it was being towed.
Capt Watson said the crew of the Bob Barker - the conservation group's newest vessel to take to the waters - tried all day yesterday to salvage the Ady Gil.
But attempts to tow the badly-damaged vessel failed and the crew was unable to keep it afloat.
Before the boat sank the crew removed all oil and fuel to avoid any pollution in the sea.
Capt Watson said the group had no other option other than to try and move the Ady Gil or abandon it.
"The Japanese harpoon vessels stayed close by but offered no assistance at any time."
Asked if they should have provided assistance, he said: "Well, they were responsible, they destroyed the vessel and the effort to try and keep it from polluting the ocean, I think, they should have offered some sort of assistance but they didn't.
"They refused to acknowledge any distress signal." One of the Ady Gil's crew members broke two ribs in the collision.
Another of the group's vessels, the Steve Irwin, deliberately stayed away to avoid being tailed by the Japanese whalers, Capt Watson said.
He said the Steve Irwin was in pursuit of another Japanese harpoon vessel, the Yushin Maru, and the crew didn't want the whaling fleet finding out their location.
The federal government has called for restraint and for both sides to respect human life.
However Capt Watson said they wouldn't restrain themselves from protecting whales.
"I think that the governments have shown so much restraint themselves over the years they've done absolutely nothing."
He also said the protesters didn't receive any call from the Australian or New Zealand authorities.
But a spokesman for Environment Minister Peter Garrett has told the ABC that neither the government nor the Australian Maritime Safety Authority received any request for help.
Greens Senator Sarah Hanson-Young told ABC Television the Japanese should pick up the bill for the Ady Gil.
"Clearly we need the investigation to happen and obviously the Deputy Prime Minister (Julia) Gillard has said there will be one," she said.
"But the big problem is that the evidence that this investigation will draw on is simply going to be taken from the Japanese because Australia was not down there."
"The Australian government have simply allowed this incident to happen by virtue of being absent in the entire activities of what's going on."
