Tradesman Matt Gee-East vividly remembers the moment he and his brothers ran to assist with a car on fire and mother-of-three Hannah Clarke, also known as Hannah Baxter, screaming by the side of the road.

Key points: Matt Gee-East says he didn't realise at first there were children in the car

Matt Gee-East says he didn't realise at first there were children in the car He saw Hannah Clarke and heard her screaming her "babies" were in the car

He saw Hannah Clarke and heard her screaming her "babies" were in the car But he says the fire was too intense to get the children out

Ms Clarke, 31, and her children — Aaliyah, 6, Laianah, 4, and Trey, 3 — died after the family car was set alight in the Brisbane suburb of Camp Hill on Wednesday morning.

Ms Clarke's estranged husband and the children's father, Rowan Baxter, is believed to have doused his family with petrol, and was found dead on a footpath with self-inflicted wounds.

Mr Gee-East was working in Camp Hill with his two brothers and a friend, building a retaining wall.

He said he heard his brother yell "fire" and thought it was a house ablaze.

They ran up the street to help and the scene confronting them was "absolutely crazy".

He said by that time, Ms Clarke was on the grass by the side of the road.

"She was just screaming out for help and then said that her babies were in the car," he said.

"I was just trying to stop the car, didn't realise the kids were in there until the door come open."

Mr Gee-East spoke slowly, breathing deeply as he remembered the moment.

"Yeah, it was bad," he said.

Hannah Clarke and her children died after being doused with petrol and set alight. ( Facebook )

"My older brother, he had known [about the children], that's why he was trying to get the door open, but it was too late.

"I didn't know what caused it either until I started hearing things on the news."

He said he saw another man "running around" at the time, but that he did not recall seeing Mr Baxter.

"Then the car started going down the hill, we tried to stop it and got the back door open, that's when I seen the kids in the car," he said.

"It's not the best of experiences."

He said by then, they could not help the three children.

"Not at that stage, no, it was too late," he said.