Ms Shortland and Tauariki had an argument before the two began punching and pulling each other's hair outside the house.
Justice Potter said Mr Leavea also punched Tauariki.
Tauariki went into the house and returned with a 32cm knife.
Justice Potter said Tauariki yelled out to Ms Shortland: "Yeah, I f***ed him, you f***ing bitch."
During the fight, Tauariki stabbed Ms Shortland twice - once in the shoulder and once in the chest.
Ms Shortland died at the scene.
Justice Potter said Tauariki took the knife out of Ms Shortland and left the property. She dumped her jacket and the knife nearby before being spotted by police.
The court also heard victim impact statements from Ms Shortland's family and Mr Leavea.
Crown prosecutor Kieran Raftery read from the statement and said Mr Leavea has "lost everything".
"I thought it would help when Ms Tauariki was found guilty of murder and in a way it did but Preva is still gone," he said.
Tauariki's lawyer Marie Dyhrberg said her client recognised the "enormous consequences" that flowed from the fight and she took full responsibility.
But she said the Crown had failed to prove that the knife had come from the kitchen.
Tauariki told the court at her trial that she had seen Ms Shortland reaching for the knife during the fight and had picked it up before she did.
Crown prosecutor Julie-Anne Kincade said that if the court was to find the knife came from the kitchen, then there was an element of premeditation in the killing.
Justice Potter said she agreed with the Crown that Tauariki had taken the knife from the kitchen. She said no one else at trial had given evidence about the knife being outside and Tauariki's story was "implausible".
