The partner of a Whangarei man shot to death during an "execution-style killing" is still wondering why he was murdered, leaving her unborn son without a father.

Otene Cook was three months pregnant when her baby's father Moses Noor Mahanga was shot at close range while sitting in a car on the night of October 4 last year on William Jones Dr near the entrance to Fishbone Park in Otangarei.

The man who pulled the trigger, Michael John Paewhenua, was found guilty of murder by a jury in December in the High Court at Whangarei following a 10-day trial.

He was yesterday sentenced to life imprisonment with a minimum non-parole period of 12 years.

Why he killed Mr Mahanga is still a mystery, although evidence at trial was that Paewhenua was agitated with his former partner and threatened to shoot people he accused her of sleeping with.

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.