Preston told Boyer he knew Ms Fan had fought whoever killed her because a Housing New Zealand employee told him so.
Preston said he and the HNZ worker sometimes swapped stories.
"She came around several weeks after I'd done it," Preston said during a phone interview.
Boyer told the court: "I said to the reporter sitting next to me: 'I think he's just confessed'."
Defence lawyer Steve Gill asked about a subsequent article published in the Weekend Herald on March 29, 2014.
Mr Gill said the article reported that Preston maintained his innocence, and Preston's denials formed the "tenor" of the story.
Boyer said the article never mentioned the "I did it" line because of an editorial decision.
He said it was deemed unlikely the comment would have made it to print past the Herald's legal team.
With deadline looming, it was decided to cut the legally risky quote from the story.
Boyer said he emailed police with the "I did it" quote and others from Preston, asking for comment.
Mr Gill said Boyer did not know what was going on in Preston's mind and couldn't exclude the chance Preston's comment was "just a slip of the tongue".
Detective Senior Sergeant John van den Heuvel said at the time police were keeping an open mind about who killed Ms Fan.
Preston told Boyer, in a conversation played to the court, that "every single finger and thumb" of Ms Fan's was cut.
He said he was the last person to see Ms Fan's body at the burial home after her death, and mentioned other wounds.
"Forgive me but I had to peek under the dress to see."
But Preston also told the reporter that when he was first arrested and bailed he "felt no fear because I knew what I knew".
After mentioning how Ms Fan fought, Preston started breaking down again. "This is my life," he said.
The trial before Justice Joseph Williams and a jury of 12 also heard several of Preston's bugged conversations with his old friend Graham Bullman.
In a conversation played to the court, Preston told his friend: "All along I have been trying to get Mei deported, and I finally successfully completed all the required [work].
"It was very cathartic for me."
"Cathartic? You've got to be f***ing joking," Mr Bullman responded.
"I had a goal ... that was my focus, and once I finally completed everything I was a happy little chappie," Preston replied.
He described to Mr Bullman accessing Ms Fan's computer using a "computer hacking password thing".
"It took well over an hour and Bingo - got into her computer and I simply downloaded everything ... I copied everything and then I deleted everything ...
"And then just to annoy her I deleted some programmes which basically f***ed her computer up," he laughed.
He said he then told his children "mummy's going to be leaving".
The Crown argues that Preston killed Ms Fan in a frenzied knife attack to gain sole custody of the couple's children.
Preston denies the charge of murder and another of breaching a protection order.
The trial continues.
