A man who the Crown alleges "just snapped" and hit his mother at least 12 times with a hammer admitted attacking her in a 111 call.

Martin Joseph Matthew Marinovich is charged with murdering Noeleen Ann Marinovich, who died on February 7 last year.

He denies the charge and is on jury trial at the High Court at Auckland, which is presided over by Justice Tracey Walker.

Today, that jury heard the 111 call Marinovich made early on February 8 while parked at a train station.

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"I can't believe what's happened," he said.

"I snapped and I hit her in the head with a hammer."

He said he thought he had tried to strangle her before hitting her with the hammer.

Questioned by the call taker, he confirmed the address where he left his mother alone.

He was unsure how long ago that was.

"I can't remember. I was in such shock."

He told the call taker he was sorry.

"I know you are ... but I need you to stay on the phone," she replied.

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She asked if he was under the influence of drugs or alcohol, which he denied, and what he planned to do at the train station. He didn't know.

When he indicated police had arrived, the call taker told him to give them a quick wave and that she would stay on the line until she heard the officers speaking.

Nearing the end of the call he repeated: "I can't believe it happened."

Two constables arrived and Martin Marinovich was arrested for assault.

The Crown alleges Martin Marinovich murdered his mother after likely arguing with her in the cramped and dirty Oratia home they shared.

"Ms Marinovich would rely a great deal on her son," Crown prosecutor Elena Mok said.

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That was because about a week before her death she was admitted to hospital after collapsing in her house, she said.

While there, she called her son often.

After she was discharged, she required help from her son, including in the bathroom.

Martin Marinovich was not socially outgoing and kept to himself in his room, Mok said.

It appeared he had few, if any, friends, she said.

"It is plain the defendant was isolated and under a great deal of strain in the lead-up to that night."

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The argument was the final straw, she said.

They might never know "what specifically prompted" the argument.

But the Crown said there could not be doubt: "He is responsible for her murder."

Crown prosecutor Elena Mok. Photo / Sam Hurley
Crown prosecutor Elena Mok. Photo / Sam Hurley

Defence lawyer Shane Tait told the jury it was accepted it happened at the hands of the defendant.

The jury needed to focus on whether the cause of death was strangulation or the blow of the hammer, he said.

The jury also needed to focus on the idea of murderous intent and whether the defendant suffered any mental illnesses, he said.

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The trial is set down for two weeks.