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Lucille Scollay, 45, denies murdering Guy Scollay, 48, at the pair's Edgeware Rd home on February 10 last year.
The cleaner pleaded not guilty at the start of her High Court jury trial in Christchurch this morning.
But she accepts that her actions resulted in the death of her husband, her defence counsel Rupert Glover said.
There are only two possible verdicts - guilty of murder or guilty of manslaughter, he said.
"So what is really on trial is not the facts of the case but the state of mind of Mrs Scollay. It's a very, very tragic case indeed."
Mrs Scollay sobbed as prosecutor Mark Zarifeh outlined the Crown case.
In the early hours of Sunday, February 10 last year, she returned home after a night out drinking with a man she had become close to.
Walking up the drive, she decided that she would kill her husband, who had been on a methadone programme, was suffering from depression, and his mental health issues were badly affecting their marriage, the Crown says.
She took a large knife from the kitchen and went into the marital bedroom where her husband of around 20 years was sleeping, it is alleged.
"She took her husband by his shoulder as he lay sleeping on his side, rolled him onto his back, got on top of him, straddling him as he was still half asleep, brought the knife up and stabbed him in chest, a deep wound that penetrated his heart," Mr Zarifeh told the jury.
With almost immediate remorse and regret, the Crown says she raised alarm with her 19-year-old son who was sleeping in his bedroom.
He called 111 but Mr Scollay could not be saved.
The wound caused immediate massive internal and external bleeding, the court heard.
"It is as a result of her actions, that she faces this charge of murder," Mr Zarifeh said.
Mr Scollay was an intelligent man, with an honours degree in history, the court heard.
But his mental health issues never allowed him to realise his potential. He was on anti-depressant medication and suffered from anxiety attacks.
He left the house rarely; only to go grocery shopping, to the chemist, or local second-hand bookstore to browse and read.
The court heard that Mrs Scollay became deeply dissatisfied and frustrated with her husband's ill mental health, and felt he would never change.
She had had a six-month affair with a man she had worked with around 10 years ago. They had started seeing each other in the fortnight leading up to the death.
The pair had been out drinking at friends' properties, parks and a pub when they returned to the Scollay house in the early hours of February 10.
They parked outside and talked, at which point Mrs Scollay became emotional and upset as she talked about her life with her husband.
"She said things to the effect she had wasted her life and couldn't see things getting any better, that Guy was really sick and not getting any better, and getting more and more difficult to live with," Mr Zarifeh said.
The man tried to calm her down before she went inside about 2.30am.
It's alleged she later told police that she felt "desperate" and "something had to happen".
"I didn't think I would do it, but I did," she allegedly told police.
Police also say she told them she decided to stab her husband as she was walking up the long drive to their rear flat.
Watching her "obviously quite brilliant husband" deteriorate over 20 years resulted in "pressure building up... to a point where she simply broke", Mr Glover said.
But she denies meaning to murder him that night.
"She simply wanted to try and get him to listen to her and to try and change their desperate, desperate lives," he said.
Mr Zarifeh said the issue for the jury was a relatively narrow one: whether the Crown can prove beyond reasonable doubt that Mrs Scollay had a murderous intent at the time of the stabbing.
The trial, before Justice Cameron Mander, is set down for four days and will hear from 19 witnesses.