Anti-whaling activist Peter Bethune has launched a tirade of criticism against the New Zealand government over its stance on whaling and its handling of his court case in Japan.

The 45-year-old was deported from Japan last week after being given a suspended sentence for illegally boarding a Japanese whaling ship.

After four months in a prison in Japan and another month in detention on board a Japanese whaling security ship, Mr Bethune is now a free man.

While he is still at times emotional about his arrest, trial and time in jail, he could see a little of the funny side.

"I was treated like Hannibal Lecter. They took me into the prison. I had 12 guards escort me to my cell," he said.

"Most inmates have a single guard or escort but I had 12. The only good thing that came out of that was I was perceived in prison as being the most evil and dangerous person by all of the other inmates who all saw this."

But Mr Bethune also remains intensely serious about his cause.

"Since 1987 when the moratorium on commercial whaling came into effect, Japan's so-called scientific research program has killed over 10,000 whales and generated over $1 billion in revenues. To call this anything other than commercial whaling is farcical," he said.

"They are speared through the back with an explosive harpoon. They then spend the last 30 minutes of their lives thrashing in agony as they slowly but surely bleed to death.

"What upsets me the most about this however is where it happens. It is in my backyard and it is in the backyard of all Aussies and Kiwis.

"That Japanese vessels sneak past us each summer to then slaughter whales in our whale sanctuary is deeply offensive."

'Fat little lap dog'

He says the New Zealand government sided with the Japanese despite his boat the Ady Gil being sliced in half by the Japanese whaling security ship the Shonan Maru 2.

Mr Bethune thanked Australia, saying the Australian Government - not his own - had shown leadership.

"The many people from all over the world who protested on my behalf, who supported me on Facebook and in forums and who raised the profile of whaling. The Government of Australia who have announced they are finally taking action against Japan in the International Court of Justice," he said.

"It is fantastic to finally see some politicians showing some backbone over this issue.

"I get the feeling these days that New Zealand has become like a fat little lap dog and any country like Japan that wants to walk over us, we are too afraid to do anything about it."

But New Zealand prime minister John Key has labelled Mr Bethune as ungrateful, saying he has received plenty of help.

"I think he's downright ungrateful. In the end he's had a sentence which has allowed him to return to New Zealand," he said.

Mr Key says he has always maintained that it would have been wrong for his government to get involved in Mr Bethune's trial in Japan.

During his trial Mr Bethune was cut adrift by the Sea Shepherd Conservation Society which he had been campaigning for. He was banned on the grounds he had been carrying a weapon.

But Sea Shepherd's Laurens De Groot, who was sitting next to him today, says it was all designed to influence the court to give him a more lenient sentence - and it worked.

"Now Peter is out of prison, we start the preparations for Operation No Compromise and again we'll take our ships down to Antarctic and we will stop the Japanese illegal whaling," Mr De Groot said.

"If there is a decision made that we needed to board another ship, then we probably will."

Mr Bethune is now back in his Sea Shepherd-branded clothing and weighing up his future.

"Man, I have sacrificed a lot and if I will be back down in Antarctica this year, I don't know but I firmly believe what Sea Shepherd stands for," he said.

"I will always be opposed to Japanese whaling and the other campaigns they do, the Blue Fin Tuna, the shark finning, those things are dear to me as well.

"If I'll be back in Antarctica this year, I don't know. I've spent a long time away from my kids."

The catch is Mr Bethune gave assurances during his trial that he would not disrupt the Japanese whaling fleet again.