A New Zealand family's investigation has led an Australian coroner to challenge the police view that their Sydney-resident daughter Nadine Haag committed suicide.

New South Wales Deputy State Coroner Paul MacMahon said he was not satisfied that 33-year-old Nadine Haag took her own life, and instead declared an open finding.

That decision was prompted by her family's private investigation, after she was found dead fully clothed with her head resting on a small green pillow in the shower of her Sydney apartment on December 3, 2009.

Police later concluded Nadine had taken her own life, after a suicide note, pills and a razor were found at the scene.

Unconvinced, Nadine's family carried out their own investigation.

Their evidence included the discovery of a note saying "he did it", a marking of a tile saying "he did it", evidence of a struggle, knives missing, a toxicology report, and Nadine's positive outlook on life.

They said Ms Haag was planning visits to see them, was attending regular gym classes, had a new romantic interest and had that morning filled her car with petrol.

Mr MacMahon acknowledged the many unusual aspects of her death.

"There was no blood found on the floor of the shower, the tiled walls ... or on any of the various items found within the shower itself."

He said the father of their young child, Nastore Guizzon's activities on December 3 and 4 also raised "considerable suspicion".

"The overwhelming evidence was that in his relationship with Nadine he was a bully who behaved in a controlling and abusive manner towards her."

He also noted the movements of Mr Guizzon on the day of Ms Haag's death, "and his subsequent evidence concerning those movements, also gave rise to real suspicion as to the possibility of his involvement in Nadine's death".

Mr Guizzon has denied any involvement in her death.

Mr MacMahon said the circumstances in which Ms Haag was found suggested intentional self-harm. There was no evidence a third party had restrained or drugged her.

But he described her as a "young, vigorous and single-minded woman" who had worked through many difficulties in her life.

"Nadine's circumstances at the time of her death are most unusual for a suicide.

"In my years as a coroner I have had to investigate many deaths that were due to suicide. In the vast majority of such cases the investigation will identify one or more aspects of the deceased's life that provides an explanation, or at least a clue, as to why they acted to end their life."

- Otago Daily Times, AAP