The parents of a New Zealand musician pushed to his death through a Perth pub window are bracing for a return to the courtroom as their son's alleged killer faces a retrial.

Andy Marshall was talking to a woman in 2011 when Stefan Pahia Schmidt shoved him through a full-length second-storey glass window at Cottesloe's Ocean Beach Hotel.

Mr Marshall is the cousin of Feilding farmer Scott Guy, who was shot dead at the end of his driveway in July 2010.

Schmidt faced trial for murder in 2012 at the same time as Mr Guy's brother-in-law Ewen Macdonald, who was found not-guilty of Mr Guy's killing.

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After sitting through Schmidt's trial, Mr Marshall's parents Alan and Wendy returned to New Zealand to support their family at Macdonald's hearing.

Schmidt, who admitted pushing 29-year-old Mr Marshall but claimed he didn't mean to kill him, was found guilty and jailed for at least 14 years.

But last year the conviction was overturned on appeal, with the Australian Court of Appeal ruling a miscarriage of justice took place because the trial judge did not adequately direct the jury on an essential element of the case.

His two-week re-trial starts in the Supreme Court of Western Australia on May 19 and Mr Marshall's parents are preparing to leave their Hawke's Bay home and again sit through a gruelling court case.

Even though they've heard the evidence before, they were unsure what to expect this time round. They do know it will be difficult.

"You try to keep your mind open and deal with it one day at a time. You never stop being a parent. You're always a parent in life and death," Alan Marshall told APNZ today.

"You don't stop feeling or loving your own children and I guess that's the price you pay in some ways for that relationship and that depth of love."

However, that depth of feeling would also help them through their ordeal, along with support and prayers from family and friends.

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Alan Marshall also expected more focus this time on his son's injuries and pathology evidence, as was eluded to in the appeal court ruling.

It was recently decided that the re-trial would be before a judge only, without a jury, something Alan Marshall was initially disappointed to hear.

"But I guess we just have to accept it and there's probably some points in favour in the sense it takes the theatrics out of it to some degree. It might also be a shorter trial. I know last time there was a few delays for jurors."

When in Perth the Marshalls will stay in an apartment near the court, and planned to do a lot of walking to relax.

"We take our walking shoes. There's quite a few of Andy's friends over there so we will touch base with them ... I've also got a niece and her husband over there and their little one."

Andy Marshall was a drummer in a band called Rich Widow and would have turned 32 in December.