Cook said she and Mr Mahanga were in a relationship for two years until the unimaginable happened that broke her heart into a million pieces.
"I screamed, I cried, I had no self-control. Even when I saw him after he died, it still felt like a dream," she said in a victim impact statement read to the court during sentencing.
"To MJ (Paewhenua), I want you to know you have not only taken a life, you have ruined other people's lives. Worst of all, you've made my son fatherless. You've taken Mo's life and nobody knows why. It seems a totally senseless killing," she said.
Justice Rebecca Edwards said Paewhenua's actions were senseless.
She said the aggravating features of his offending was the use of a sawn-off shotgun from close range, directing the shot at Mahanga's head which meant he had no prospect of survival, and fleeing the scene.
"It was a deliberate act to kill. It was an execution-style murder."
Paewhenua was on bail when he shot dead Mr Mahanga. Justice Edwards said the use of a firearm and the nature of the killing required greater denunciation by the court.
Crown prosecutor Bernadette O'Connor said since 2006, Paewhenua had consistently offended with a propensity towards violence. Paewhenua's defence at trial was he did not kill Mr Mahanga, whom he considered a friend.
Yesterday the court also remitted Paewhenua's outstanding fines of $9599.