In her opening address this afternoon, Crown prosecutor Emma Light spoke of a "fractious" relationship peppered with violent outbursts.
The relationship spiralled out of control from late 2012, she said, nine years after the pair met when Preston was in China looking for work.
A "revelation" Ms Fan had been seeing Tani Hoyhtya, a United Nations worker based in Southeast Asia, sent the relationship into freefall, Ms Light said.
In 2013, Preston was focused on trying to remove Ms Fan from New Zealand -- his goal to get sole custody of the two children, she said.
"The defendant discussed his campaign to have Ms Fan deported with a number of witnesses..." Ms Light said.
Those witnesses would be among the roughly 110 people the Crown would call to give evidence over the course of the trial, she added.
Preston took "the desperate and callous step of killing his estranged wife to secure custody of their two children", Ms Light said.
He tried covering his tracks after killing Ms Fan -- but he wasn't as smart as he believed, Ms Light said.
"Unhappily for the defendant, if he thought himself forensically aware", he was mistaken, she said.
Preston used a wig and changed his clothes on the day of the murder, the Crown alleged, but a witness saw him driving past with the wig on.
The jury was told black polypropylene fibres were tangled round the knife found in Ms Fan's body, and the Crown suggested these "fake hair" fibres came from the wig Preston wore.
The same wig was attached to a hat one of his children wore on Hallowe'en just a few days earlier, prosecutors said.
The bloodied knife, an image of which was shown to the court, was still embedded in Ms Fan's neck when she was found on November 10.
There was no evidence of a burglary, she said, and Ms Fan's tablet, mobile phone and laptop were all left at the Brussels St home.
The Crown would rely on a circumstantial case, but when "a number of strands" were connected, they would prove Preston's guilt, Ms Light said.
However, defence counsel Louise Sziranyi said her client was "not guilty of either charge".
"Mr Preston has maintained his innocence from the 10th of November 2013 and still does to this very day," she said in a brief opening address.
It was "extremely important" jurors kept an open mind, Ms Sziranyi said.
Matthew Breach, who found Ms Fan's body, told the jury he received a text from Mr Hoyhtya on November 10, 2013, saying Ms Fan had not been heard from for some time.
"I pray she is okay and alive. Let me know," Mr Hoyhtya told Mr Breach. "I am really worried," he added later.
Mr Breach called cellphone numbers for Ms Fan twice, but got no response.
He went to Ms Fan's house, finding the door locked.
"On closer inspection...I saw what may have been blood or dark brown stuff on the handle."
Mr Breach inspected the exterior of the house, looking through the windows and a ranch slider.
"I could see Mei or [who] I thought was Mei, lying face down...she was lying there face down with blood around her," he said. "It was quite evident that a knife was in her neck."
Mr Breach told neighbours. "I just said, 'I think she's dead'."
Senior Constable Tony Milner was one of the first officers at the scene.
He and another officer, Rory Spray, armed themselves with Glock pistols and approached the address.
Mr Milner said he observed what looked like "blood spatter" on the inside panel of a door.
He looked though another window and saw the top portion of a body with a knife "embedded" in the neck.
After entering the house, Mr Milner could not tell the race or sex of the body and said a pool of blood was around the victim.
Detective Olivia Gibson of Wellington Central police was later put in charge of looking after the body.
She said the tip of the knife was sticking out of the Ms Fan's jaw, her left hand was under her forehead, and her right hand was wedged under the laundry door.
The trial, before Justice Joe Williams, continues tomorrow, and is expected to last up to five weeks.
