Former Greens leader and Sea Shepherd head Bob Brown has slammed the Abbott government for failing to send a ship to monitor Japan’s whale hunt.

The Abbott government is being accused of turning its back on Australian law by breaking an election promise to send a Customs vessel to monitor Japanese whalers in Antarctica.

The government will send a plane to observe protesters and whalers in the Southern Ocean, not the purpose-built Customs ice-breaker vessel Oceanic Protector.

It’s the first monitoring mission on Antarctic whaling in six years, but conservationists say a plane will be useless and the government has reneged on its commitment to send a ship.

“It’s a plane cop out,” Bob Brown, Sea Shepherd Australia chairman, told AAP on Monday.

“They’re going to fly over at a height seeing what we all know is happening down there, and that’s the bloody and illegal destruction of Australian whales.”

Greg Hunt, the Environment Minister, said the flights will send a message that the Australian government expects all parties to abide by the laws of the seas.

But Mr Brown said the $150 million Oceanic Protector, which is patrolling at Christmas Island thousands of nautical miles away, should be on the job.

“The government has got it to turn back the [asylum seeker] boats, but not the Japanese whaling boats,” he said.

He said the minister “who should stand up for Australia’s rights” had caved under pressure and, “weak at the knees” and “kowtowing”, didn’t want to offend Japan during free-trade negotiations.

The former federal Greens leader said the government hadn’t been willing to send a vessel since the federal court in 2008 ruled Japan’s hunt illegal and issued an injunction against it.

But by not taking action, Prime Minister Tony Abbott was effectively instructing Australia’s policing authorities to “turn their back on this huge breach of Australian law”.

Cabinet member Steve Ciobo said it would be more effective to monitor and report back what is happening from the sky than engage in a potential stand-off with Japan on the sea.

“This way we can get an overarching view of what’s taking place,” he told Sky News on Monday.

But Mr Brown said: “That plane is not going to prevent anything. Who are they going to call, and from where, and how long is it going to take to get there?”

Sea Shepherd has three ships and around 100 crew – including 50 Australians – heading to Antarctica to confront Japanese hunting in the whale sanctuary.