Graphic footage of Japanese whalers hunting in an ocean sanctuary has been released after a five-year legal battle.

The Australian government refused to release the video at the request of marine conservation charity Sea Shepherd in 2012, due to fears it would damage relations with Japan.

But the footage, which was filmed by customs officials in 2008, has finally been handed to the charity after the country's Information Commissioner ordered customs officials to release it.

The video shows a Japanese “research” vessel slaying minke whales. It captures the moment one is hit by an explosive harpoon before being reeled in to the side of a ship and held in place until it dies.

Jeff Hansen, Sea Shepherd’s managing director, said the whales were being hunted in an Australian Southern Ocean whale sanctuary by harpoon ships travelling up to 24 knots, faster than the whales can swim.

“Eventually they come up exhausted and when these whales are hit by an explosive harpoon, it sends shrapnel through their bodies and then hooks come out,” he said.

“So there is no way for these whales to escape. They’re thrashing around, trying to get away from these harpoon ships, but this cable drags them back up to the surface. It takes a long time for these whales to die.”

Once dead, they are tied up to the side of a harpoon ship before being dragged up the slipway of a factory ship and being “diced up for the markets in Tokyo”.

Amazing aerial views capture bowhead whales in their natural habitat

Describing the practice as “barbaric”, Mr Hansen said hunting continues in the sanctuary to this day and he urged Australia's government to take action or risk Japanese whalers continuing to hunt 10 years from now in the country's waters.

“Does the Australian government represent the wishes of the Australian public, who want to see an end to whaling, or Tokyo?” he said. “It’s time the Australian government stepped up and sent a vessel south.”

The attempts of the Australian government to keep the footage secret have sparked accusations it prioritised diplomatic considerations over animal welfare and the wishes of its citizens.

Sea Shepherd said it no longer has the resources to send out protection boats and is calling for the government to take Japan to the International Tribunal for the Law of the Sea.

SeaWorld's killer whales Show all 10 1 /10 SeaWorld's killer whales SeaWorld's killer whales Visitors are greeted by an Orca killer whale as they attend a show featuring the whales during a visit to the animal theme park SeaWorld in San Diego, California March 19, 2014. A California lawmaker introduced a bill to ban live performances and captive breeding of killer whales in the state, a measure that would force the SeaWorld San Diego marine theme park to end is popular "Shamu" shows. Mike Blake/Reuters SeaWorld's killer whales A trainer shows the crowd a killer whale during a show at the animal theme park SeaWorld in San Diego, California March 19, 2014. A California lawmaker introduced a bill to ban live performances and captive breeding of killer whales in the state, a measure that would force the SeaWorld San Diego marine theme park to end is popular "Shamu" shows. Mike Blake/Reuters SeaWorld's killer whales An Orca killer whale is seen underwater at the animal theme park SeaWorld in San Diego, California March 19, 2014. A California lawmaker introduced a bill to ban live performances and captive breeding of killer whales in the state, a measure that would force the SeaWorld San Diego marine theme park to end is popular "Shamu" shows. Mike Blake/Reuters SeaWorld's killer whales Visitors get a close-up view of an Orca killer whale during a visit to the animal theme park SeaWorld in San Diego, California March 19, 2014. A California lawmaker introduced a bill to ban live performances and captive breeding of killer whales in the state, a measure that would force the SeaWorld San Diego marine theme park to end is popular "Shamu" shows. Mike Blake/Reuters SeaWorld's killer whales Trainers have Orca killer whales perform for the crowd during a show at the animal theme park SeaWorld in San Diego, California March 19, 2014. A California lawmaker introduced a bill to ban live performances and captive breeding of killer whales in the state, a measure that would force the SeaWorld San Diego marine theme park to end is popular "Shamu" shows. Mike Blake/Reuters SeaWorld's killer whales Trainers have Orca killer whales perform for the crowd during a show at the animal theme park SeaWorld in San Diego, California March 19, 2014. A California lawmaker introduced a bill to ban live performances and captive breeding of killer whales in the state, a measure that would force the SeaWorld San Diego marine theme park to end is popular "Shamu" shows. Mike Blake/Reuters SeaWorld's killer whales Trainers have Orca killer whales perform for the crowd during a show at the animal theme park SeaWorld in San Diego, California March 19, 2014. A California lawmaker introduced a bill to ban live performances and captive breeding of killer whales in the state, a measure that would force the SeaWorld San Diego marine theme park to end is popular "Shamu" shows. Mike Blake/Reuters SeaWorld's killer whales Trainers have Orca killer whales perform for the crowd during a show at the animal theme park SeaWorld in San Diego, California March 19, 2014. A California lawmaker introduced a bill to ban live performances and captive breeding of killer whales in the state, a measure that would force the SeaWorld San Diego marine theme park to end is popular "Shamu" shows. Mike Blake/Reuters SeaWorld's killer whales Tillikum, a killer whale at SeaWorld amusement park, performs during the show "Believe" in Orlando, September 3, 2009. A killer whale at the SeaWorld amusement park in central Florida killed a trainer on February 24, 2010, police and company executives said. According to the Orlando Sentinel the orca involved in the incident, named Tillikum but popularly known as "Tilly," has a controversial past. Mathieu Belanger/Reuters SeaWorld's killer whales An unidentified trainer works with a killer whale during the "Believe" show at Sea World in Orlando, Florida, in this photograph taken on February 14, 2010. A killer whale at the SeaWorld amusement park in central Florida killed a trainer on February 24, 2010, police and company executives said. Picture taken February 14. Richard Baum/Reuters

Josh Frydenberg, Australian environment minister, told the Sydney Morning Herald that the government was "deeply disappointed that Japan has decided to return to the Southern Ocean this summer to undertake so-called 'scientific' whaling as it is not necessary to kill whales in order to study them".

He added: "The government will continue to advocate for Japan to comply with its international obligations and the principles set out in the International Court of Justice's judgment."

Japanese hunting ships departed for the Southern Ocean earlier this month.