Hannah Baxter died at the Royal Brisbane and Women's Hospital early on Wednesday evening.
Her husband Rowan Baxter died alongside his children Laianah, Aaliyah and Trey on a quiet suburban street in Brisbane's east on Wednesday morning.
A member of the New Zealand Warriors NRL squad in the mid-2000s, Mr Baxter had been working at a Brisbane fitness centre with his wife, who was a trampolining champion.
Police said it was too early to tell whether it was murder-suicide or an accident, although one witness told News Corp Ms Baxter had run from the car screaming, "He's poured petrol on me."
Neighbours reported hearing multiple explosions before seeing the car engulfed in flames in Raven Street at Camp Hill about 8.30am on Wednesday.
Another man who tried to help at the scene was also taken to hospital with facial burns.
The Baxters were owners of a fitness centre but had reportedly separated late last year.
The family was not involved in any Family Court or Federal Court proceedings.
Detectives are still trying to establish how the fire started.
Mr Baxter's Facebook profile is littered with pictures and videos of the three children, along with comments he "loves them to the moon and back" and "daddy misses you all".
Six days ago his friend Joey Abraham wrote under a photo of little 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)"
One witness told News Corp Ms Baxter ran from the car screaming, "He's poured petrol on me".
However, it is too early to draw any conclusions, according to Detective Inspector Mark Thompson.
"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, it's inappropriate at this stage," he said.
"I've seen some horrific scenes - this is up there with some of the best of them.
"It's a terrible thing to be presented with."
