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An expert witness’ “twist of evidence” has today raised doubts, defence lawyers say, over whether missing man Michael McGrath was at home during a one-hour window that it’s alleged his prison guard mate shot him dead several kilometres away.

Electricity consumption records at McGrath’s house in the Christchurch suburb of Halswell have come under scrutiny as the trial of David Benbow, who denies murder, enters its sixth week at the High Court in Christchurch.

The Crown alleges that Benbow, 54, lured his builder friend McGrath to his Candys Rd home at 9am on May 22, 2017, to help him move some heavy railway sleepers. It says Benbow shot him dead before attending a counselling session in the city at 10.15am and later dumping his body.

McGrath has never been found and nor has any murder weapon.

During its opening address to the jury, the Crown said they know McGrath was alive that morning because power “spiked” at his Checketts Ave home between 8.30am and 9am as he made his breakfast and got ready for the day.

But after he allegedly left for Benbow’s about 9am, there was no significant power use “indicating that no one was home after that time”.

Genesis Energy employee Catherine Mace earlier said power consumption at McGrath’s house between 8.30am and 9am, suggested it could have been someone going about their normal morning business.

Christchurch builder Michael Craig McGrath, 49, was last seen at his home in Halswell, Christchurch, in May 2017.

The electricity meter at the house was subject to automation, the court heard, with power going on and off at intervals throughout that day.

But she was recalled to the witness stand this morning, saying that she had made some further inquiries and discovered she had made a mistake.

She said the time bracket was now for 9am-9.30am - and that she had apologised to police for her error.

Under cross-examination by one of Benbow’s lawyers, Kirsten Gray, Mace said the two scenarios for the energy use were either someone going about their normal business or hot water heating up.

“Are you trying to help the Crown out a bit now?” Gray said to Mace.

“No. I didn’t mention the hot water prior because I didn’t think it was necessary,” she replied.

“I wasn’t in the house so I don’t know what was going on between that period of time.”

Gray reminded Mace that, as an expert witness, she must be impartial and called it a “twist of evidence”, with the jury learning for the first time today that the energy spike could have been hot water warming up.

The Crown called its final witness this morning, the officer in charge of the Operation Renovation case, Detective Inspector Kylie Schaare.

During questions from defence counsel Marc Corlett KC, it emerged that police employed a specialist company’s ground penetrating radar to look for human remains or buried evidence at Candys Rd.

The move came after someone who bought the property from Benbow had “raised some concerns”, the court heard.

Three areas of potential interest were identified and excavated in January 2020 but nothing was found.

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The trial is ongoing at the High Court in Christchurch. Photo / Kai Schwoerer, Stuff, Pool

After the Crown closed its case around 12.30pm today, Corlett confirmed that Benbow will not give evidence.

He has nothing to add to hours of interviews with detectives, his lawyer said.

However, the defence will call several witnesses over the next 2-3 days.

Addressing the jury, Corlett said while he wouldn’t “respond or delight in all the deficiencies in the Crown case” until his closing comments, but he couldn’t pass without comment on the “shambles” of today’s evidence.

The expert evidence of Mace figuring out she was wrong about the power use timings means the police theory collapses, the lawyer said, and led to “the entire premise of the Crown case [being] flawed”.

He reminded the jury that the Crown case has McGrath driving his Subaru to Benbow’s house at 8.54am and he “can’t be in two places at the same time”.

Later, Corlett says he will explore the “cascading effect” of that premise being wrong, he told the jury.

He earlier warned the jury of “investigative bias” and “tunnel vision” from police early in its investigations, claiming they were trying to find evidence to support their case.

The Crown, however, allege that Benbow murdered McGrath just weeks after learning he was seeing his ex-partner Joanna Green and telling a counsellor he wanted to “annihilate” him.

While the Crown accepts there is no body, no murder weapon, and little forensic evidence in the case, it says a strong circumstantial case consists of many threads that, when taken together, show Benbow is guilty of McGrath’s murder beyond reasonable doubt.

The trial, before Justice Jonathan Eaton, continues.