Lady Heeni said it would be difficult to come to terms with his death. "I've got lots of wonderful memories and he's just an amazing man."
Sir Peter's children and some mutual friends were with her at the house, she said.
"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."
Lady Heeni said close friends had been coming to visit Sir Peter.
"In the last two to three weeks we've had group readings around here ... friends would come around and we would read a particular book out aloud to Sir Peter, because with the way the cancer had developed, his eyes became tired and he wasn't reading at all."
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.
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 defended in more than 100 murder trials over his career, said at the time 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 had campaigned tirelessly for prisoner rights, advocating for decent conditions, 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 more alternative residential facilities where offenders could undertake programmes aimed at changing their behaviour.
He defended in some of New Zealand's highest-profile trials, including Arthur Allan Thomas, Terry "Mr Asia" Clark, and in the Bassett Rd machinegun murders case.
Sir Peter was also a keen sailor and an author. He most recently wrote The Dwarf Who Moved, about his long career.