The 48-year-old Invercargill police officer who executed his wife before trying to kill her new partner will spend at least 17 years in prison.

Constable Ben McLean pleaded guilty last Thursday to murdering his wife of 18 years, Verity McLean on Anzac Day.

The mother of their three children died from a single gunshot to the head, less than three weeks after she told McLean she was leaving him for family friend, Garry Duggan.

McLean also pleaded guilty to attempting to murder Duggan.

In the High Court in Invercargill this morning, the 48-year-old police officer offered an apology to his three children, as well as the wider McLean, Barber and Duggan families.

The two struggled and Duggan wrestled the rifle from McLean. McLean handed himself into the Invercargill police station, where he worked, 40 minutes after the shooting.

On April 5 this year his wife Verity McLean, 40, known as "Bert", had told the defendant she had begun a relationship with Dugan, his close friend of 15 years.

On Anzac Day, after his wife had broken into the Scott St address they formerly shared to retrieve her belongings, he packed a bag and cycled to her new abode.

His rucksack contained a cut-down .22 rifle to which a silencer had been fixed with black insulation tape.

And there was further evidence of McLean's preparation.

He packed blue disposable rubber gloves, a bottle of hand sanitiser, plastic handcuffs (the same type the Armed Offenders Squad use), strips of towel and a portable radio tuned to the Invercargill police channel.

Verity McLean was found dead with a single bullet wound which had entered under her chin.

The scene at Otepuni Ave in Invercargill after off-duty policeman Ben McLean shot his estranged wife and her new partner. (Photo / Kurt Bayer)

Strips of towel later found in McLean's bag were bloodstained and covered in saliva, a summary of facts said, and inside a fireplace were burned remains consistent with plastic handcuffs.

Duggan arrived back at the Otepuni Ave property at 8pm, where McLean lay in wait.

He was faced with his former friend training a rifle directly at him.

McLean shot the victim in the right upper chest and again in the forearm.

Despite the wounds, Duggan grabbed the firearm and smashed it over McLean's head, causing the stock to break off.

A violent struggle ensued as the man fought for his life.

But McLean pulled out a 50cm wooden baton from the backpack and hit him in the head.

McLean put the victim in a "choke hold" but he continued to fight back, using the baton against his attacker.

Eventually, the exhausted pair reached a physical stalemate and sat arguing.

When Duggan quizzed him about Verity McLean's whereabouts, the defendant claimed she was at a friend's house and soon after fled on his bike.

Police arrived to find Duggan bleeding profusely and Verity McLean dead in the house.

Later that night McLean was driven to the police station and handed himself in.