Neighbour Andrew Sinclair,who lives five houses away from where the car went up in flames,said it was a “traumatic” and “awful” incident to witness.
“I unfortunately saw the mother and her suffering. There was a lady who came out with a hose to put over her[Hannah’s] horrific burns,” Mr Sinclair said.
“I heard what I thought was a smoke detector and I thought it was a standard neighbour’s toaster going off,then I heard a grinding sound and some sort of car alarm.
“I saw the flames coming out of a car … then as I got closer I saw the lady,Hannah,and saw what was being done.
“They were horrific burns. It’s sad that she has passed,but the pain she would have been suffering mentally and physically would have been horrific. She was conscious during this time.
“Nothing could be done,the flames were so intense. Twenty metres away you could feel it as if you were beside a campfire.”
Theconfronting scene shocked emergency workers,who were called to Raven Street just before 8.30am to find the car engulfed in flames.
"I've seen some horrific scenes,this is up there with ... them,"Detective Inspector Mark Thompson said near the scene.
"How the fire actually occurred has not been ascertained at the moment,so for us to call it a murder-suicide or a tragic accident is inappropriate for us to try and do that."
Friends of Ms Clarke paid tribute to the victims on social media,describing the loss as a"nightmare".
"These beautiful kids will forever be remembered. My heart breaks for you Hannah,"one wrote on Facebook.
"She was such an incredible mum,I just can’t fathom that this has happened,"another said.
Rowan Baxter once played for the New Zealand Warriors rugby league club in a pre-season match against NRL club the Parramatta Eels. He had previously played as a winger for Bay of Plenty in the New Zealand rugby competition.
Photos of the children,including one of Trey with his grandfather on rowing machines,had previously been shared online by the Baxters'business,Integr8&CrossFit.
The couple reportedly separated late last year and were trying to work out custody arrangements. However,they were not involved in any proceedings in the Family Court or Federal Court.
Ms Clarke was a trampolining champion who represented Queensland for four consecutive years.
The couple's Integr8 fitness website described her as"an enthusiastic,passionate mother of three".
The website said Mr Baxter had trained AFL,Super Rugby and NRL players.
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Five days ago friend Joey Abraham wrote under a photo of Trey:"Chin up brother everything will work out just hang in there my bro alot of people care about you and the situation your facing (sic)."
Emergency services remained at the cordoned-off block into Wednesday evening as the vehicle was towed away and clean-up efforts began.
A collection of flowers and plush bears grew on a nearby street corner throughout the afternoon as neighbours and numerous politicians,many holding back tears,visited the site.
One neighbour,who did not give a name,said her husband woke her about 8.30am when he smelt"burning rubber"from their home two blocks away. She said she heard a helicopter about 30 minutes later.
"We walk our dogs up this street,"she said.
Neighbour Murray Campbell heard multiple explosions from his backyard and came running out the front to see the car,which he did not recognise,"engulfed in flames".
"[There were] multiple explosions ... it was very loud,like a gas cylinder exploding. It was every bit as bad as that."
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-With AAP