A major defence witness in the Michael McGrath murder trial has backflipped on his own key evidence with new information coming to light while he was on the witness stand.
Power expert Ron Beatty started his evidence on Wednesday by saying McGrath must have been at home and alive when the Crown alleges he was shot at David Benbow's but he backpedalled on Thursday because of new information provided by the Crown.
David Benbow entered court less than a week out from when the jury is expected to decide his fate.
The Crown alleged CCTV footage shown of Michael McGrath in his car on the morning of May 22 heading to David Benbow's house just before 9am, soon to be shot dead by him, meant he couldn't have been at his home at Checketts Ave, in Christchurch's Halswell.
On Wednesday Beatty who analysed McGrath's electricity usage blew the Crown's case apart.
"My conclusion is that Mr McGrath was home definitely home until 9:30am," Beatty said.
He dismissed the idea that it could have been the automated hot water cylinder reheating.
"It must have been Mr McGrath using electricity actually within his premises," Beatty said in court on Wednesday.
On Thursday Beatty's evidence changed when Crown Prosecutor Barnaby Hawes asked him "that one kilowatt could be accounted for by Mr McGrath being home or not being at home and you can't say?"
Beatty said, "that's correct."
That's because new data supplied by the crown showed the water cylinder could've reheated within the crucial time plausibly explaining the power spike.
Beatty conceded: "The fact that electricity was consumed or not is not a good indicator on its own actually of the property actually being occupied."
The witness also told the court he had no idea how crucial the timing issue was for this trial, that is, if McGrath was home making breakfast. He can't have been in the process of being murdered five minutes away at his mate's house.
When one of the lawyers asked if he was aware of the timing issue in the trial Beatty said he wasn't aware it was.
"I didn't actually understand the actual significance of that," Beatty said.
He still stuck by his evidence that he believes someone was in McGrath's house after 9am that day, because of two peaks in electricity after then.
Neither Michael McGrath's body nor a murder weapon have ever been found.
