Weeks before allegedly killing his boxing teacher, Sean Hayde strangled and assaulted his former partner after goading her with details of his affair, a jury has heard.

Hayde and Gregory Hart are on trial in the Auckland High Court charged with murdering Wiremu Arapo and setting fire to his Cockle Bay home to destroy the evidence of their alleged killing.

Police have also charged Hayde with assaulting, strangling and threatening to kill his former partner, who gave evidence on Tuesday, the second day of the trial.

Hayde and Hart, who have known each other since their early teens, are pointing the finger at each other.

Each will claim the other was solely responsible for beating Arapo to death on October 20, 2020.

Arapo was described as a well-liked and athletic former soldier turned personal trainer with a fiancé, with much to look forward to.

The Crown’s case is the killing was the work of both Hayde and Hart.

Crown prosecutor Ned Fletcher said it was preceded by a tangled web of deteriorating relationships and infidelities, with the first eruption of violence being Hayde’s alleged assault of his former partner.

 “This is a case about relationships, relationships gone wrong, and their snowballing effects,” Fletcher said.

Sean Hayde is standing trial at the Auckland High Court, accused of the murder of Wiremu Arapo in Cockle Bay, whose body was found after a house fire. Fellow accused Gregory Hart not pictured. Photo / Jason Oxenham

Arapo was Hayde’s boxing tutor.

Hayde had met another woman via Arapo and the pair had begun an affair, the Crown says.

Through Hayde, Arapo met Hart, who then moved into the Cockle Bay flat Arapo shared with his fiancé, and where he operated a personal training business.

While Arapo was initially happy to play the role of matchmaker, he came to take a dim view of Hayde’s budding relationship with his friend after he learned of the alleged assault of his former partner, the Crown claims.

For his part, Hayde began to resent the fact his new partner and Arapo remained close, the jury heard.

Hart was characterised by Fletcher as a layabout flatmate frequently behind with the rent. The day before the alleged murder he went out to buy alcohol and cigarettes instead of paying the power bill.

The tensions then reached their crescendo with Hayde and Hart driving around to Arapo’s property and beating him to death, Fletcher alleged.

They then staged the scene to make it look like the fire had been caused by cannabis “spotting” on a gas stove gone wrong, then burned down Arapo’s wing of the home, the Crown claims.

On Tuesday, the Crown called Hayde’s former partner, the victim of the alleged assault and strangulation.

The pair met online in 2019 and began dating. She was employed but Hayde did not work, the jury heard.

The couple moved in together to a home in Bucklands Beach Rd in August 2020.

Gregory Hart (left) is jointly charged with murder and attempting to pervert the course of justice. Photo / Jason Oxenham

Around this time, the woman began to hear more and more about the female friend Hayde had met through Arapo.

Her suspicions grew, especially after it emerged the woman had been visiting their home, she said.

“I didn’t trust her as far as I could throw her and the fact she kept coming to my house really upset me.”

On August 29 Hayde admitted he was sleeping with the woman, his ex-partner said.

Far from being apologetic or trying to downplay what happened, he goaded her with details of the affair, the jury heard.

“It was just humiliating,” the ex-partner said.

The day after the revelation, Hayde suggested they have a party to celebrate their break up.

“He was just being very cruel towards me.”

Crown prosecutor Ned Fletcher said the case against Hart and Hayde is about relationships, and how they soured before the alleged eruption of violence and fire. Photo / Jason Oxenham

Over the next two days Hayde allegedly strangled the woman, and repeatedly assaulted and threatened to kill her.

He was arrested, charged and bailed for the alleged domestic violence, a few weeks before the alleged murder of Arapo.

The jury watched police interviews with the ex-partner where she shook as she described details of the alleged assault.

She said she had told him she just wanted to get back her effects and leave the house.

Hayde then poured hot sauce over her head and told her she wasn’t going anywhere, the woman told the police officer.

He then threatened to kill her and said he was going to set their house on fire and prevent her leaving, she said.

Paul Borich KC is representing Hart alongside Jonathan Hudson. Photo / Jason Oxenham

After he poured the hot sauce onto her hair, he began singing “You are worthless, no one’s ever gonna love you, ha, ha, ha, ha, ha,” the woman said.

While she was on the toilet, Hayde pulled her by her hair on to the floor and snapped her phone, she told the officer.

The jury watched another police interview with the woman from August 2020 where she described being strangled by Hayde to the point of losing consciousness.

Hayde had her in a chokehold for about 20 seconds, until she passed out.

When she regained conciousness, Hart and Hayde were standing over her, she told the officer. Hart said “she’s all right bro” while Hayde said “that’s for your own good”, the woman said.

The trial continues.