Within days of discovering his wife was leaving him, an Invercargill police officer was searching rifles for sale online.

When Constable Benjamin Peter McLean, 48, pleaded guilty to murder and attempted murder before the High Court at Invercargill yesterday, court documents were released which suggested he meticulously planned the shooting of the woman and her new partner.

On April 5, Verity McLean, 40, known as "Bert", had told the defendant she had begun a relationship with his close friend of 15 years, 47-year-old Garry Duggan.

Seven days later he had searched online for .22 rifles for sale; "cheap".

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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.

"Some aspects suggest it may have been staged to resemble a suicide," a police summary said.

But a post-mortem refuted any possibility the death was self-inflicted.

It revealed nearly 40 other injuries consistent with "a fight or fracas", the court heard yesterday.

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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 evening, McLean called his children telling one of them: "I have f***ed everything. I'm f***ed. I went round and shot and killed Bert and shot Garry ... I'm on the run".

He told them: "I've killed her. I don't know why I thought it was a good idea".

I went round and shot and killed Bert and shot Garry ... I'm on the run.

After calling his daughter, her boyfriend and his mother took McLean to the police station to hand himself in.

On the way there, McLean told them his kids "would hate him forever and he would be inside for at least 20 years".

When Verity McLean's father visited his daughter's killer at Invercargill Prison on May 3, he explained his actions.

"She wouldn't talk to me or have anything to do with me and I snapped," McLean told him.

The pair had been married for 18 years, according to court documents, and they had three children together.

Verity McLean and Duggan had started seeing each other late last year. It took her more than four months to break the news to McLean.

The victims' families requested privacy after yesterday's hearing.

"This has been a very difficult year for us all and as Christmas approaches we are all going to find it a tough time but will get through it together with the support of those around us and community understanding," they said in a statement.

Justice Rachel Dunningham remanded McLean in custody. He will be sentenced on Monday.

READ MORE: Policeman Ben McLean admits murdering his wife after she left him for friend