A lick of paint and modern carpet were the only changes to the Coogee apartment in Sydney's sought-after eastern suburbs that looked like it had been frozen in time for more than 40 years.

So, when detectives peeled back the carpet last month, they found blood from the cold case murder still stained the concrete.

Investigators are now hopeful forensic analysis of the blood using technologies that were not around 40 years ago may help them finally identify the person responsible for one of the city's most horrific unsolved murders.

Detectives believe 26-year-old Mrs White's killer may have cut himself during the frenzied attack, leaving blood traces behind.

"We know her blood is going to be there obviously," Detective Senior Constable Deon Kelly, the officer in charge, said.

"But even if there is a minute amount of his blood that we can somehow get, then that is reason enough to go back [to the crime scene]."

Mrs White was at home with her newborn baby, Shane, on June 8, 1973, after her husband Paul left for work at the St George Pictorial in Hurstville.

At 10am she called her father to remind him it was her mother's birthday. About midday she answered a knock at the door.

Once inside, the man terrorised Mrs White, forcing her at knifepoint to undress in front of him and to fold her clothes in a neat pile.

His sadistic intentions were disturbed when someone delivering nappies turned up outside, prompting Mrs White to run towards the front door, presumably for help.

Her killer followed, stabbing her at least 11 times and slitting her throat in the hallway.

Mrs White fell back into the second bedroom, where Shane lay in his cot.

This is how her husband found her when he arrived home to find the door slightly ajar at 7.30pm.

"I just went straight into the bedroom, turned the light on and there was Lyn just lying there," Mr White said from his home in Sydney's south this week.

"Shane was in his cot about a metre away, there was blood everywhere."

Mr White first laid eyes on Lynette, a caring woman with a great sense of humour, at Grace Bros at Bondi Junction in the early 1970s.

They were married for two years before the killer burst into their home, possibly under the guise of asking for a glass of water.

Ten months later, on April 22, 1974, 20-year-old Maria Smith was raped, gagged and strangled after answering a knock at the door of her Randwick home.

Similarities between the crimes brought the two cases under the same investigation – Strike Force Tourmaline.

The cases were reinvigorated early last year after NSW Police received valuable information that a notorious serial killer could have been behind the murders.

Detectives recently visited Bandali Debs in Goulburn prison, where he is serving life sentences for the murders of two Victorian police officers and two prostitutes.

He was happy to talk but told police he was in Europe when Ms Smith was murdered, a claim later confirmed.

"This is one of those hard slogs and about going down every possible avenue," Detective Chief Inspector John Lehman, head of the Unsolved Homicide Unit, said.

"For example, we have persons of interest and we have seen these people were either not considered back in the day or weren't looked at probably closely enough and were discarded too early in the investigation.

"That is why we are now trying to catch up and make sure those persons are looked at more closely."

Working on the theory that the killer hurt himself during Mrs White's murder, police are cautiously hopeful that the discovery of blood at the scene may provide the breakthrough her family need.

When police arrived at the Coogee unit in 1973, they found the nappies that had been left at the front door had been brought inside and were soaked in blood.

"The theory was because of those nappies he brought back in that he may have injured himself, potentially badly," Detective Kelly said.

"Because it was such a frenzied attack, it is possible the knife slipped."

Preliminary results from the blood found under the carpet showed a male DNA profile mixed with Ms White's DNA profile.

Police are running the profile through a national DNA database for potential matches.

The development may help mitigate some challenges posed to the case because of missing post-mortem swabs and original crime scene samples.

It also could bring Mr White closer to finding out who killed Lyn.

"I will never give up until I know where he is," he said.

"No way will I forget but that will give me a chance to move on."