Respected and sometimes controversial New Zealand lawyer Sir Peter Williams has died.

The 80-year-old Queen's Counsel, who has battled with prostate cancer for almost a decade, died at his Auckland home this evening.

His wife Heeni Phillips-Williams said he died about 6.30pm tonight surrounded by friends and family in their Ponsonby home.

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She said it would be difficult to come to terms with his passing.

"We had some wonderful times together."

She said Peter's children were at the house with her, as well as some mutual friends.

"We're sort of shocked because we didn't expect him to go downhill so quickly.

"He has had blood transfusions, two or three of them, and the last one was last Tuesday at Auckland Hospital and then when he came home he was just losing more and more strength and so he's been in bed since last Tuesday."

Sir Peter was one of New Zealand's most respected legal minds and a prison reform campaigner.

He was never battle-shy, having fought for his clients in the courtroom for 60 years, and campaigned for prisoner rights.

READ MORE:
Twelve Questions with Sir Peter Williams

The QC was made a knight companion of the New Zealand Order of Merit for services to law in the New Year Honours.

Sir Peter, who has conducted more than 100 murder trials over his career, said he felt "very humbled" and "very privileged" by the honour.

He received his knighthood early, at a special ceremony in April at his Ponsonby home, due to his ailing health.

Since being admitted to the bar in 1960, Sir Peter campaigned tirelessly for prisoner rights, advocating for decent conditions during incarceration, rehabilitation and addressing the causes of crime.

Presiding over the Howard League for Penal Reform for 30 years, he resigned in 2011 to establish the Prison Reform Society.

He argued for less focus on imprisonment and more on alternative residential facilities where offenders could do programmes aimed at changing their behaviour.

Sir Peter took on clients in some of New Zealand's most significant trials, including Arthur Allan Thomas, Terry "Mr Asia" Clark, and in the Bassett Rd machine-gun murders case.

Sir Peter, who was diagnosed with prostate cancer in 2006, said he felt it was his duty to represent those who wanted his services.

He was also an author and most recently worked on the book The Dwarf that Moved, which contained stories from his long legal career.

Sir Peter said he had lost count of how many murder trials he had served on, but estimated the number was between 100 to 200.

In a revealing 2012 interview with the New Zealand Herald, Sir Peter candidly discussed his cancer diagnosis.

He said his doctor recommended brachytherapy, in which radioactive seeds were implanted in the prostate gland. He suffered harsh side-effects.

"Within a very short time I had to use a catheter. I had to use that for quite a long time and it was very painful and exacerbating. But after that things seemed to be okay ... but then a couple of years ago the cancer re-emerged.

"The doctors put me on hormone tablets to attack the testosterone which feeds cancer, so the idea is that if you get rid of the testosterone the cancer diminishes."

Sir Peter said that treatment helped, but he also underwent radiation therapy at Mercy Hospital.

"Administration is painless - the after-effects are extreme tiredness, also nausea and feeling very uncomfortable. All you want to do is sleep. You've got to labour on. Having coffee with friends and keep involved in what's going on."

Sir Peter also stayed active and swam in the mornings.

"I'm active. I work in the mornings," he said in the 2012 interview.

Lady Heeni said Sir Peter obviously knew he was going today because he called those who had not been to see him.

"Like our two dogs, who came on to the bed and sat with him and myself."

Sir Peter's two dogs were named Kafka and Araki.