The sentence given to the Khmer Rouge henchman who headed the prison where New Zealander Kerry Hamill was tortured and killed is not enough, his brother says.

New Zealand champion rower Rob Hamill was in court today when Kaing Guek Eav, better known as Duch, was sentenced to 35 years jail. However that sentence was reduced for time the 67-year-old has already served.

Hamill said he was initially pleased.

"And I thought yep that's good he'll probably be there for the rest of his days."

But then he realised Duch would only be behind bars 19 more years and would be about 86 when released.

"Definitely not appropriate. The possibility of this guy walking free at any point in the future I just don't believe is appropriate," he told Radio New Zealand.

He said 35 years was acceptable given a maximum 40 was allowed under international law.

"I was initially pleased because the judges only mitigated five years of that 40 and that is a reasonably pleasing outcome considering, but at the end of the day he could be a free man."

Duch is the first Khmer Rouge cadre to be convicted by an international tribunal over the deaths of up to two million people through starvation, overwork and execution at the hands of the regime.

Kerry Hamill ended up at the S-21 or Tuol Sleng prison when the yacht he and friends were sailing strayed into Cambodian waters on August 13, 1978.

One crewman, Canadian Stuart Glass, was shot while Mr Hamill and Briton John Dewhirst were taken for interrogation and torture for two months before being killed.

Mr Hamill testified at the Extraordinary Chamber of the Courts of Cambodia (ECCC) on July 26 last year - the 31st anniversary of his brother's abduction. The ECCC is a joint Cambodia-United Nations court and former New Zealand Governor-General Dame Silvia Cartwright is one of the judges.

Led by Pol Pot, who died in 1998, the Khmer Rouge sought to set up a communist utopia. Up to two million people died from starvation, overwork, torture or execution during the 1975-1979 regime.

"It's the end of a chapter with an exclamation mark," said Mr Hamill. "This was a big deal, an historic moment for humanity but in terms of internal family grieving processes... it never ends."

The family struggled with Kerry's loss, one brother committed suicide and Hamill found himself drinking too much. His parents never really got over it.

Hamill is still not letting go. Before sentencing, he bailed up Duch's lawyer to try to arrange a meeting which may happen in the next few days.

"I just want to understand him."

He wants to question Duch about how his ideology led him to kill people and why he carried on even when it must have become clear Cambodia was no communist utopia.

He also wants to press for more information about Kerry, whose body was never found and whose photo - taken of all prisoners on arrival - went missing.

Despite not being happy with the length of the sentence, he hopes it will give some solace, especially to Cambodians who fled the regime.

"I'm hopeful they will get something from it. At least one person has been bought to some sort of justice... To know that this man who met my brother and all those other people in that prison is now sentenced is some sort of justice."

Hamill testified during Duch's hearing and is participating in a documentary.

"Bringing up painful memories isn't necessarily a bad thing," he said. Not having Kerry's body and a lack of information about what happened to him had hampered the grieving process.

The country too, was broken, Hamill said.

"People here are so held back, there's still fear attached to it all. It's a broken society, it really is."