Rowan Baxter kept a rope in his car to murder his ex-partner and son from a previous relationship, years before killing his wife Hannah Clarke and their three kids.

Clarke and her children Aaliyah, 6, Laianah, 4, and Trey, 3, were killed last week when Baxter doused them in petrol and set them alight in a car after ambushing them on the school run. He then took his own life.

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Before Baxter married Clarke, he was in a relationship with another woman and had a son, Isaiah, who is now in his early 20s. Baxter told Clarke that when the earlier relationship ended, he was going to kill the woman and his son, according to his second wife's closest friend, Nikki Brooks.

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Tragically Isaiah, left, pictured with his three half siblings and dad Rowan, says he was also abused. Photo / Facebook
Tragically Isaiah, left, pictured with his three half siblings and dad Rowan, says he was also abused. Photo / Facebook

"Hannah had told me that he had a rope and something in his car,'' Brooks told The Australian.

"He'd driven to do it. He'd prepared what he needed to kill Isaiah and (Isaiah's mother) and then didn't do it. Isaiah would have been maybe 7 or 8. It was the first time she was going to leave him as well.''

Baxter told Clarke he had almost killed someone else in a separate incident when Isaiah was a baby, resulting in a conviction for grievous bodily harm, the newspaper reports.

"Hannah just knew there was a road rage incident — he lost his mind and nearly beat someone to death," Brooks said.

Hannah Clarke's parents Lloyd and Suzanne Clarke and her brother Nathaniel speak out after murder. Video / 9 News

"He would make it out to be someone else's fault, of course — someone did something and he acted the way he did because he had a baby in the car.''

After years of abuse, Clarke separated from Baxter in November last year and the two had been working on custody arrangements.

"We said, 'Han, enough is enough,'" Brooks told A Current Affair.

"It was getting bad and we had to get her out of there."

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Former rugby league player Rowan Baxter, originally from Tauranga with his wife Hannah and children. Photo / Supplied
Former rugby league player Rowan Baxter, originally from Tauranga with his wife Hannah and children. Photo / Supplied

Brooks said when Clarke hid at her house following the separation, she seemed "happy" and "relieved".

"She stayed with me and we felt safe. He didn't know where I lived. She just looked relieved and she just seemed really happy. She knew she made the right decision," she said.

Clarke's brother Nathaniel spoke to the ABC's 7.30 programme on Tuesday, saying his sister had received "amazing support from the police".

What "cut" him the deepest, he said, was that Baxter had made his wife and kids suffer before their deaths.

"It wasn't quick. It was planned and executed," he said.

"He had a plan that night when he called the kids and he was a blubbering mess. He knew what he was doing then. He had it all planned out, he knew what he was doing the following morning.

"He couldn't even do it quick. That's the worst thing. He made them suffer, and her."

Baxter's cousin Alana Hampson, who lived with him in New Zealand after she ran away from home as a teen, said he had a dark past.

"He was a monster, even when he was a kid, I think he was a monster … He was very controlling and manipulative," Hampson told Newshub.

"I didn't like to be around him because I felt uncomfortable. The way Rowan used to speak to his mother was … disrespectful."

Photo / Supplied
Photo / Supplied

Hampson said Baxter had an obsession with sex and treated women with contempt from a very young age.

"I think that Rowan should have been going through counselling as a kid," she said.

"The kids got into porn, always looking at porn [and] his mum accepted it, he treated his mum like c**p. She was a nice lady."