The stepbrother of Grace Millane's murderer says the guilty verdict - and hearing the gruesome details of her death - have been "incredibly hard" for the family.

He first spoke to the Herald days after the then 26-year-old was charged with murder last December.

This evening he said he "shed a few tears" when he heard the verdict.

"So glad for Grace's family," he told the Herald.

"It doesn't make anything better, but [it's] good to know the jury and New Zealand justice system made the right call."

He said his family had struggled through the trial and found it "divisive" but had good support.

It was hard hearing the intimate details of his sibling's sex life - and about what he did to Millane and how he treated her body afterwards.

"It's been incredibly hard for the family to come to terms with the enormity of everything," he said.

The stepbrother said he was shocked when he first learned of the crime and initially thought it could have been an accident.

But his view changed dramatically when he heard the facts presented in court.

The murderer's father attended much of the trial.

As the jury deliberated he sat alone, waiting, looking drawn and anxious.

He did not speak to media after the verdict was given in court, during which he remained calm and collected, glancing at his son - now a convicted killer.

The stepbrother said he had reached out to his stepfather.

He was the only member of the family in Auckland for the trial.

"Understandably he will be in pieces right now," the stepbrother said.

He said the wider family were coping as best as they could.

He also spoke to One News about the trial and said his sibling was a "pathological liar".

"[He] lies over pointless things and continues to lie until the point where he's got no out, absolutely no out, and then he breaks downs and cries and runs away," the stepbrother said.

"But he can't do it any more, thankfully, just absolutely terrible that a life had to be lost because of that."

Still he doesn't believe his stepbrother had set out to kill Millane.

"It's in that moment, yes he had the opportunity and he kept going and took Grace's life. I don't think it's a violence thing. To me, I think it was power. It was just in a split moment, he enjoyed the power and kept going."

He believed the tears that were shed by his stepbrother in front of police were more "tears for himself" than for Millane.

The stepbrother also delivered a personal message to the Millane family.

"I'm just so incredibly sorry for their loss. To know it's one of our family members, even though it's not from our actions, is very difficult and I can't imagine the pain and hurt and what they've got to go through court, hearing all the stuff about BDSM and all the actions... it's just so ridiculous," he said.

Grace Millane. Photo / Supplied

"And to me that's because he doesn't have any shred of a decent human being inside of him and couldn't just confess to the fact that he murdered her."