On the morning of January 22, Beerens was intoxicated and formed the intention to kill the man, Crown prosecutor Chris Lange said.
She travelled from her Halswell home to his work, checking if he was there.
When she saw his car, she drove to his Riccarton flat, where one of his flatmates let her inside.
In his bedroom, she found notes and gifts from the other woman, laid them on his bed, and burned them with a cigarette lighter.
"She stood and watched the fire take hold," Lange said.
Beerens then took a black flick knife, that she knew he regularly sharpened, drove back to his work, and then returned to his flat to wait for him to come home.
The Fire Service arrived to tackle the fire, and the man soon showed up, alerted to the blaze by his flatmates.
She approached him at the front door, holding the knife in her right hand.
"In one motion, she struck the victim in the chest with the knife, driving the knife into his chest as far as the blade would allow," Lange said.
She removed the knife and dropped it into the garden nearby.
The victim dropped to the ground instantly, clutching his chest and bleeding heavily.
Firefighters restrained Beerens until police arrived.
The victim was rushed to hospital. The knife narrowly missed his heart and he underwent surgery for his injuries.
When spoken to by police, Beerens "expressed her frustration that she had failed" in her attempt to kill the victim.
She refused to tell police the reasons behind her deadly intent.
Today, Judge Cameron Mander sentenced Beerens to six years' imprisonment with no minimum non-parole period.
