The family of a woman found dead in her Sydney apartment almost four years ago are calling for a full homicide investigation into her death following a coroner's recent open finding.

Nadine Haag's body was found at her Castle Hill apartment in December 2009. Her wrist had been slashed and there was a razor blade, pills and a note near her body.

Police believed it was suicide but the Haag family never believed the 33-year-old had taken her own life, so they embarked on a long and difficult search for the truth.

Their efforts led to an inquest and the New South Wales deputy coroner handed down an open finding earlier this month, saying Nadine Haag's death was a mystery.

But the family says they have uncovered compelling evidence that points to a brutal murder and are urging police to do the detective work that was not done back in 2009.

The Haags sought the advice of former Victorian homicide inspector Lucio Rovis. To him, Nadine's death looked like a staged suicide scenario.

"I have been to many suicides and murders where an artery has been severed and the amount of blood is dramatic at the scene," he told the ABC's 7.30 program.

"So I think that what has happened is that whoever has inflicted the injury on her has probably done it in the shower, but on exiting the shower has left these smears and drops on cleaning up."

Nadine was a fitness instructor and devoted mother. Cherie Haag said her sister had everything to live for.

"Nadine was a beautiful, bubbly, funny person, she loved life, she loved her family," she said.

"She was the person who would get up and go, very motivated, very caring she was just a beautiful person, she would walk into the room and make everyone smile."

The Haag family are convinced Nadine's did not take her own life.

"We went to see Nadine in the morgue and observed bruising and marks on her body, and we knew straight away that this was not suicide. It looked like she had been beaten," Cherie Haag said.

The Haag family strongly believe Nadine was killed at the hands of her ex-partner, Nastore Guizzon. The couple had been in the midst of a custody dispute over their daughter.

"I have seen him and his tempers, I know of his outbursts, I have seen his outbursts, I have seen Nadine become very afraid of him in my presence, I know what he is like, I know how he is," Nadine's mother Linda Sansoni said.

Nadine's family set about tracing Mr Guizzon's movements at the time of her death.

He claimed he had been home all day, nowhere near Nadine's apartment in Castle Hill, but his phone records tell a different story.

"The phone records, they are fact, they are empirical," Mr Rovis said.

"They show that at the time Nestore said he was at home, in fact he wasn't, he was in the vicinity of Nadine's apartment.

"The next day when he said he was at work early in the morning, in fact he wasn't, and he was still at home.

"The e-tag records for his mother's car also show the same discrepancies, so they are built on lies, not realising that they could actually be looked at and prove their statements to be lies."

The family made another astonishing discovery during an interview with a female NSW Police officer.

"We were told that inside the evidence bag with the first note was a second note, and so the detective took it out of the bag, it was a screwed up piece of paper and semi-folded," Cherie Haag said.

"We were told there was just scribble on the paper, because on the back there was scribble, like an 'h'.

"As she unfolded it it revealed the words 'he did it' and the detective was rather surprised that it revealed those words."

Cherie Haag said the police had no idea the note was there. The family believe Nadine wrote it and had tried to hide it.

Two years after Nadine's death new tenants in the apartment found another message scrawled on a tile in the bathroom: the same words, 'he did it'.

At the end of the inquest deputy state coroner Paul MacMahon said he would not make a finding of suicide. In relation to Mr Guizzon, he found that his:

"...actions on 4 December 2009 and his subsequent evidence concerning those actions certainly add to the suspicion that he had an involvement in the death of Nadine.

"I am satisfied that Guizzon had a motive to harm Nadine, had the opportunity to do so and lied about his whereabouts on 3 and 4 December 2009."

But he said there was no evidence that placed Mr Guizzon in the apartment at any relevant time.

Mr MacMahon said parts of the evidence given by Mr Guizzon and his mother Velonika Guizzon, who he lives with, were untrue.

The ABC offered Mr Guizzon the opportunity to tell us his side of the story. He declined.

The 7.30 program also visited the family home in Brisbane to seek their response to the inquest findings. Ms Guizzon said she had no comment to make.

The coroner chose not to refer the matter to the Director of Public Prosecutions (DPP).

Mr MacMahon said it would not be possible to establish beyond reasonable doubt that a known person caused Nadine's death.

The family's barrister, Penny Wass, said the open finding was a "fantastic result".

"It leaves the door open for the NSW Police to do what they should have done and to have a proper homicide squad have a look at it," she said.

A spokesman for New South Wales Police told 7.30 the force was currently conducting a review of the coroner's findings and seeking further advice from the homicide squad.

The Haags say they have come this far and will not rest until they find out exactly what happened to Nadine and prove it.

"It has destroyed our lives, not only have we had to suffer the loss of a loved one - a mother, a sister, a daughter - and then to have to fight just to get little bits of evidence, little bits of information, trying to piece this together - nobody should have to go through this," Cherie Haag said.

"We know the truth, the truth has kept us going. We knew eventually somehow, sometime the truth would get us to where we are today.

"We have still got a long way to go but it's the truth."