"We still can't get our heads around it," Mr Marshall said.
He found the grieving process was a difficult thing to explain.
It was tough to talk about but the couple said they wanted to speak out in the hope their faith and determination to continue life as normally as they could would help others faced with great loss.
"Talking is so important," Mr Marshall said.
"Talk to each other - talk to family and to people outside the family. It can be heartbreaking sometimes but it is important."
Mrs Marshall said there were "terrible" days when she had to search deep to find the courage just to get out of bed.
"Your energy just disappears. You have to take life minute by minute then hour by hour and day by day."
Andy Marshall was a sports and music-loving adventurer. He was a young man who loved to travel and meet people and make new friends - and he made many.
"Just a good kid - a really good kid," Mr Marshall said quietly.
His son had been to Europe several years ago and was intending to go again, but this time via Perth.
But the economic situation in Europe collapsed as the recession struck so he stayed in Western Australia for what would turn out to be two-and-a-half years.
He'd come home a couple of times to see family and friends, and on Mothers Day in May 2011 he called his mum for a chat.
It was the last time they spoke - that night he died after being pushed through a glass window by a man who would later go on to be charged and convicted of murder.
The man's life sentence was important to the family as it meant he was out of society.
"But it's no real closure - it doesn't change anything," Mrs Marshall said.
"But we have to have hope - we have other children (daughter Katie and another son Ben) and there is a grandchild on the way. You go on."
Mr Marshall took the fateful call at 9am on a Monday morning from a detective in Western Australia.
"It is the worst call you can get. He said who he was and that he was with the Western Australian Police. He asked if I was the father of Andrew Marshall ... and I knew something was very wrong."
The officer said "I have some very bad news for you" and life suddenly went into a numb freefall.
Mr Marshall then had to make the worst phone call he would ever make - to his wife who was at work.
"When he told me I felt like collapsing," Mrs Marshall said.
They were living in Tauranga then although their daughter Katie was living in Napier and at the time was awaiting surgery in hospital.
They drove down to tell her.
Ben, who was living in Christchurch, had already heard the news from Perth and had seen the incident on social media before his father called him.
"We're a close family - it was a massive loss," Mr Marshall said.
They began the long-distance calls and arrangements to get his body back to New Zealand, and travelled to Perth to collect his possessions.
They had planned to go over to see him at some stage and regret they never did.
"We were met at the airport there by the media ... not Andy."
They cleared out his flat, sold his car and brought some belongings home, including his bike.
"I get out and ride it now," Mr Marshall said.
"Me and Andy together," he added with a smile.
The family returned for the trial of 26-year-old Stefan Schmidt who had links to an outlaw motorcycle gang, and later spoke to the court via a video link from Hastings.
They heard how their son had been talking to two girls with his back to the window at the Ocean Beach Hotel when Schmidt swore at him and pushed him through it.
As Schmidt fled the pub he briefly glanced at Mr Marshall, who he did not know, as he lay dying on the footpath.
Mrs Marshall described the loss and having to deal with it as like being in "an avalanche of black snow".
They will never cease to grieve, but said family and faith got them through, and would continue to get them through.
"We believe we will see Andy again," she said.
"Andy is still in our family and he is part of our lives - and he brought a lot to our lives."
The memories are strong and cherished.
A typical boy, his room was often a bit of a mess, and he loved music - especially the drums. He drummed for a couple of bands in Christchurch and while in Perth.
He loved his golf, fishing and cricket.
To honour their boy they commissioned the Andrew Marshall Memorial Trophy for bowling which they presented to Riccarton High School in Christchurch late last year.
"He loved adventure and he'd try anything," Mr Marshall said.
"He laughed heaps - you'd always hear him before you saw him and he wouldn't know how to make an enemy. He just made friends - lots of them."
Mr Marshall and the couple's son-in-law Daniel, who is a builder, built a bench seat which is "Andy's Seat" and embedded in the armrest are a set of the drumsticks he once used.
With a smile, Mr Marshall told of how after hearing thunder, the 3-year-old son of one of his son's mates in Christchurch told his dad that "Andy's in heaven playing the drums".
"We heard some thunder ourselves a little while back," Mrs Marshall said.
"And we said 'Yes, Andy's playing drums again."
A jury found Stefan Pahia Schmidt guilty of murdering Andy Marshall on June 20 last year, after an eight-day trial.
In September, he was sentenced by the Supreme Court of Western Australia to life in prison and will serve a minimum of 14 years.
His lawyers have appealed the sentence and this is set to be heard next month.
