A Japanese vessel tagged by authorities as responsible for wrecking a ship inside Australian waters in 2009 has reportedly re-entered the country's territorial zone, reports said.

Sydney Morning Herald reported on Wednesday that Shonan Maru No.2 has been spotted within the confines of the Australian Exclusive Economic Zone (AEEZ), allegedly closely monitoring the movements of Sea Shepherd ships, which are being used by activists to disrupt the whaling activities of Japanese ships.

Tokyo sanctions seasonal whaling in open seas under the auspices of its Institute of Cetacean Research.

The country maintains that the exercise was mainly for scientific purpose but activists have long criticised the Japanese government for protecting the activities that they accused were mere guise to commercial whaling.

As of Wednesday morning, Shonan Maru, the SMH wrote, was last seen tailing the activist group ships and already within 50 nautical miles of the AEEZ, near the cost off Western Australia.

According to Sea Shepherd chief Paul Watson, the security ship is expected to tail next the Steve Irwin ship once the Brigitte Bardot heads to dock for much needed repairs.

This was the same ship that harassed Steve Irwin on December, Watson said, and was earlier identified by activists as the vessel that destroyed the Ady Gil some two years ago.

"This ship refused to co-operate with Australian and New Zealand authorities after destroying the Ady Gil ... it would seem to me that they would be wanted for contempt if nothing else," Watson told SMH.

As part of Japan's whaling expedition, authorities said that Shonan Maru is technically prohibited from entering the territorial waters of Australia and New Zealand.

According to Watson, officials of the whaling security ship seem unaware of the international laws and even entered the AEEZ bounds without any effort of concealing its presence when it emitted its automatic ship identification signal.

Also, the Japanese ship, Sea Shepherd reported, is now emblazoned with the mark 'Government of Japan' when it was spotted by the activist group.