A Brisbane mother who was set alight with her three young children "did everything she could to protect" them from her estranged husband, a friend says.

Key points: Rowan Baxter and his 31-year-old wife Hannah Clarke both made it out of the burning car

Rowan Baxter and his 31-year-old wife Hannah Clarke both made it out of the burning car The three children died in the car and Hannah died in hospital early on Wednesday night

The three children died in the car and Hannah died in hospital early on Wednesday night Friends of the couple told the ABC the Baxters' lives were unravelling

Hannah Clarke, 31, also known as Hannah Baxter, was pulled alive from the driver's seat of the family car as it was engulfed by flames on a Camp Hill street about 8:30am on Wednesday, during what should have been a routine school run.

Her children — Aaliyah, 6, Laianah, 4 and Trey, 3 — all died inside the car.

Ms Clarke died hours later in hospital.

Her estranged husband and the children's father, former Warriors rugby league player Rowan Baxter, allegedly poured petrol on his family in a suspected domestic violence incident.

Police said he was in the front passenger seat but made it out of the burning vehicle.

He died with self-inflicted wounds on the footpath, police said.

Friends of the couple have told the ABC that signs of the Baxters' lives unravelling were there. ( Facebook )

Friend Caitlin Langford said Ms Baxter was "just the most beautiful woman".

"She was kind, and strong, and lit up any room," she said.

"She was the most beautiful mama to her babies and the most inspirational role model."

Flowers have been left near where the family died in the Brisbane suburb of Camp Hill. ( ABC News: Kate McKenna )

Hannah and Mr Baxter, 42, ran a fitness business at Capalaba for about five years. Ms Clarke was a champion in trampoline sports and specialised in kids and "Mums n' bubs" classes.

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The gym shut its doors late last year.

Friends of the couple told the ABC that signs of the Baxters' lives unravelling were there.

Ms Clarke had moved out with the children, and Mr Baxter was living alone in the three-bedroom Carindale home the family had shared.

Hannah Clarke with her son Trey, who died yesterday in Brisbane. ( Facebook )

Friends said problems in the marriage had been visible for some time.

"She loved them so fiercely and she was doing everything she could to protect them," Ms Langford said.

For the months after the separation, Mr Baxter peppered Facebook with messages wishing his children goodnight, posting pictures of them cuddling together, and telling them he missed them.

A fundraising page has been set up for Ms Clarke and the children's funeral.

Police are expecting to give more details of their investigation into the suspected murder-suicide this afternoon.

'It doesn't feel real'

Korri Lauder, who has known Ms Clarke for more than a decade, said her friend was "the definition of love".

"It's not just her family that's crushed it's the entire trampolining community and gymnastics Australia," she said.

Korri Loader reacts at the makeshift shrine near the scene of the car fire. ( AAP: Dan Peled )

"It doesn't feel real, it doesn't feel like it's her.

"She lived, breathed [her kids]. She'd do anything for those kids.

"When you think of family, you just think of her."

Rowan Baxter posted video of himself rough-housing with children

Mr Baxter (on left), used to train with the NRL club New Zealand Warriors. ( Facebook: Rowan Baxter )

Mr Baxter, born in New Zealand, proclaimed he was "one of the top fitness coaches in Brisbane", and his expertise was once bolstered by a video testimonial from rugby league star Sam Thaiday.

But it was another online video of Mr Baxter that provoked the attention of many in the wake of the tragedy.

Last month, he posted a video on Facebook of him play-fighting with his children.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 58 seconds 58 s Rowan Baxter plays a wrestling game with his three children.

He responded to their playful "attacks" with practised demonstrations of the kind of headlocks used in mixed martial arts, and laughed out loud after slamming toddler Trey's face into a mattress.

"Sweet dreams my babies xo Love you to the moon and back #nowords #dad #myworld", he posted.

Others read something more sinister into the muscular man's rough-housing with the young children.

In the wake of yesterday's tragedy, more than 700 people piled onto Mr Baxter's page to post mostly damning comments under the video.

Arguments raged about whether his handling of the kids in the video represented some kind of warning sign.

The suspected killings of Ms Clarke and her children echo a series of disturbing cases more than four years ago which thrust the protection of women at risk from former partners onto the state political agenda.

Hannah Clarke and Rowan Baxter on their wedding day. ( Facebook )

The public outcry in response to a cluster of highly-publicised homicides in 2015, including Tara Brown and Karina Lock, prompted moves to overhaul police and government responses to family violence.

The Palaszczuk Government fast-tracked recommendations from Quentin Bryce's Not Now, Not Ever report.

Local MP Joe Kelly was among those who laid flowers at the scene and said he had been "sickened" to hear the news.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 31 seconds 31 s Former Broncos player Sam Thaiday trained with Rowan Baxter at his gym.

"I think this would would rock any community," he said.

"People are being killed by domestic violence every week in Australia and sadly it's now our community's turn.

"We have to continue to work really hard to stop people dying.

"To do that we have to change our attitudes and culture and we have to make sure that we treat people with respect and relationships are based on collaboration, not control."