And if a teenage boy could kill his mum so viciously over something possibly as simple as an "argument over not being able to drive the family car", then what's to stop him killing again?

Former detective sergeant Doherty examined the crime scene all those years ago and collected the forensic evidence that led a jury to convict De Gruchy on three counts of murder.

Now, 23 years later and the high probability the triple murderer could soon be a free man, the seasoned cop is left with a feeling of unease.

"It was such a strong case, there was no doubt in any of our minds that he was the one who did it," Mr Doherty said of the team who solved the case.

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"For me, the fact that he has never ever admitted to it, to be able to do something like that over something so small ... I wouldn't want to be living anywhere near the guy if he gets out," he said.

"It's one of those ones where you look at it ... if he does get out and something happens down the track, I'd be worried that could happen.

"The injuries to his mum were horrific. We thought when we first got there that the mother was shot with a firearm, that's how bad her injuries were.

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"We couldn't do a visual identification, that's how bad. When you think about it, if he's done something like this over something so trivial ... if something made him snap down the track you wouldn't know what he would do."

It was a Wednesday morning in 1996 when De Gruchy ran outside the Albion Park home "in a bad way", frantically calling for help. His neighbour Steven Bailey got the shock of his life when he entered the De Gruchy home.

Inside, Jennifer, 41, was lying on bloodstained sheets in her bedroom. Sarah, 13, was in her bedroom with massive head and facial injuries. Adrian, 15, was lying in a pool of blood in the garage.

De Gruchy's dad Wayne has stood by his son who could be released after his probation and parole hearing in June. Police never understood how or why.

Mr Doherty, who worked in forensics for 20 of his 24 years as a police officer, said the De Gruchy crime scene is one that he can't shake from his thoughts.

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"It's one of those jobs I could go back and redraw the scene sketches," he said. "Some of those jobs get etched in your mind. That's one of them.

"We spent days out there going through everything so minutely," he said. "It's probably one of the only murders I've done where the case was predominantly circumstantial. But it was an extremely strong circumstantial case."

Crucial to the case was small blood stains behind Jennifer's bed, up on the wall.

"They were out of place so we collected those. They ended up being Matthew's. Then there was the missing pieces of carpet, then some other blood smears through the house on certain items, including the light switch in Matthew's bedroom.

"When we examined the car I found a single fibre that appeared to be the same sort of carpet from inside. On that was a minute piece of what appeared to be blood. We were never able to positively group it. But we do know it did not come from the victims but it could come from Matthew. That was a handy finding."

Mr Doherty said the evidence pointed early to Matthew. "The story that he provided, it wasn't fitting," Mr Doherty said. "When we got there it had been made to look like a break-in.

"But there were things of value left in the house. Money left in wallets. He had grabbed stuff to make it look like a break-in. I've been to hundreds of break-ins, this looked staged."

Mr Doherty and his team spent days at the crime scene, then the post-mortem examinations and were on hand for the discovery of items from the family home dumped in a Woonona dam.

"This case was one we were heavily invested in. We did so much work. Danny Sharkey (lead investigator) and the guys did an absolutely wonderful job. As forensic guys we were there to support Danny and his investigative team.

"But because of our system De Gruchy is probably going to get out. For someone who doesn't admit they did it, how can they be rehabilitated? When he was held to account he still wouldn't admit it."