Hannah Clarke had difficulty seeing the violent nature of her relationship with former partner, according to a friend.

Clarke and her three children - Aaliyah, 6, Laianah, 4, and Trey, 3 - died on Wednesday after the car they were travelling in was doused in petrol and set alight. Her estranged husband, New Zealander Rowan Baxter, allegedly caused the incident before stabbing himself to death at the scene.

A friend of the fitness coach, Manja Whaley, said Clarke was in an emotionally, sexually and financially abusive relationship, but hadn't realised it.

Facebook Rowan Baxter and Hannah Clarke pictured with their three children.

"For such a long time she didn't believe she was in a [domestic violence] relationship. It hadn't crossed her mind, because as she said to me, her words 'he didn't hit me'," Whaley told Australia's Today.

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She said Baxter would often control what Clarke wore, monitor her social media accounts, accuse her of flirting and dressing proactively, and threaten to punish her and the children.

Facebook Aaliyah, 6, Laianah, 4, and Trey, 3 died on Wednesday

"Some of the things that she would explain was this excessive control ... the sexual abuse daily, and if she wasn't to have sex with him, he would punish not only her but also the chidlren."

Whaley told the morning television programme that she wasn't allowed to go to the gym in shorts, and the children weren't allowed to go to the beach on weekends.

Clarke's parents and brother also spoke with Nine's A Current Affair. Her mum Suzanne said not all domestic abuse was physical and explained how their daughter "sacrificed everything" for her children. She also noted that Baxter phoned the children the night before the tragedy.

In the wake of her death, the family was launching a campaign titled Small Steps for Hannah.

At the end of 2019, Clarke moved into her parent's house in Carindale with her children. Baxter maintained contact with her but only saw them for visits, as he didn't have primary custody.

Domestic violence advocates highlighted this was a critical moment for women leaving abusive and dangerous relationships, as it was common for the man to lash out.

Before the tragic incident, which shocked and saddened the country, Clarke had been looking forward to starting over with her children. Her brother, Nat Clarke, said the last thing she told his wife was that she was "so excited, this year will be great".

One of Clarke's previous Instagram posts began circulating after her death. In the caption, she spoke of her strength and confidence.

"I am a strong woman, I don't sit around feeling sorry for myself, nor will I ever let anyone mistreat me again.

At the end of 2019, Hannah Clarke moved into her parent's house in Carindale with her children.

"I am in control of my life and there is nothing I can't achieve," the post read.

The scene of the fire shocked first responders and witnesses who rushed to help after the car erupted in flames. Many spoke of seeing Clarke leap out of the burning vehicle and scream: "He's poured petrol on me". She was taken to hospital but later died from her injuries.

It's believed Clarke was driving her children to school at the time of the incident and Baxter had been in the passenger seat.

Australian police were yet to confirm the cause of the fire and the investigation was ongoing.