She slipped the sedative Phenergan into the 47-year-old delivery driver's evening meal and, while he was heavily sedated, suffocated him.
Milner then made his death, on May 4, 2009, look like suicide in the hope of cashing in his $250,000 life insurance policy.
"You hear of women who rip off their husband before a break-up, but to suddenly click it's okay to commit murder?
"I just can't get my head around it," says Ms Cartier, 44, who has been praised by police for her detective work in bringing Milner to justice.
Police originally ruled the death was suicide. It was only when a coroner raised doubts that a full homicide investigation was started.
Ms Cartier became sceptical within days of the funeral about Milner's story that Mr Nisbet killed himself.
From her home in Queensland, she did her own detective work, supplying information to police, and even ringing Milner's work colleagues, who told her that she had often spoken of getting rid of her husband and asked if rat poison would work.
They even nicknamed her the Black Widow.
The High Court trial, which ended just before Christmas, has taken a toll on Mr Nisbet's family, Ms Cartier and Milner's own sons Adam and Greg, who testified against her.
"Everyone says, 'You won, you won,' but it's not a win. You can never win this. Even if she dies in prison, Phil is dead and nothing will bring him back.
"All we can do is make sure she doesn't do it to anyone else."
Ms Cartier says she is desperate to find out how she can get into prison to see Milner. "I just want to scream at her.
"I need to know what was she thinking. It's tearing me up inside, not knowing. Was she out one day getting her hair done, or driving down the road, when she suddenly had an epiphany and thought, 'Yep, that's the best way out - I'll kill Phil.'
"She must have something seriously wrong with her ... I just need to know why she did it."
Milner will be sentenced on February 20.
Protocols for jail visits
Visitors to prison must fill out an application form, which is followed by security checks.
Prison staff will assess the application and make a recommendation to the prison manager.
If an application is declined, a prohibition order is issued, banning the applicant from entering the prison for a set period.
All prisoners are entitled to at least one visit each week, for a minimum of 30 minutes. The visit may be in a visiting room with other people, or in a booth.
Staff are present in the room. Visitors are allowed to give the prisoner a hug or kiss when greeting them and before leaving.
During a booth visit, there is no physical contact.
