Hannah Clarke's murderer-husband made a chilling final phone call to his three children the night before he killed them in a horrific murder-suicide.

Rowan Baxter, 42, FaceTimed Laianah, 4, Aaliyah, 6, and Trey, 3, in a "very emotional" state last Tuesday, knowing he would set them on fire and then kill himself hours later.

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Clarke's brother Nathaniel spoke about the murders on Australia's ABC 7.30 show, revealing the thing that "cut him the deepest" was knowing Baxter made them suffer to the very end.

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"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," Nathaniel said.

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

Rowan Baxter with his children in Brisbane. File photo / supplied
Rowan Baxter with his children in Brisbane. File photo / supplied

Friends said Baxter's horrific attack on his family last Wednesday had been the peak of his "unravelling"', which began when the couple separated late last year.

Clarke's best friend Lou Farmer said the situation finally "came to pieces" on Boxing Day when he kidnapped one of their daughters.

At the time, Clarke had been living with her parents and had suggested taking the kids to the park for a play date.

But just as she was packing up, Baxter suddenly grabbed Laianah and threw her in the back of his car and took off.

"Before he sped off, he said 'this is your fault Hannah', and he took her for four days," Farmer said.

Following the incident Baxter was issued a domestic violence order, which he breached just days later.

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Farmer said she had been aware of his controlling behaviour towards Clarke for quite some time, particularly the aggressive manner in which he would speak to her.

Former rugby league player Rowan Baxter originally from Tauranga with his wife Hannah and children at home in Brisbane. File photo / supplied
Former rugby league player Rowan Baxter originally from Tauranga with his wife Hannah and children at home in Brisbane. File photo / supplied

"He was one step ahead of her all the time. He knew conversations that she hadn't talked to him about, maybe us or another person. He knew of it and prompted her," Farmer added.

She said Baxter would demand sex from Clarke every night and wouldn't speak to her for days if she didn't comply.

The dad-of-three finally reached a breaking point last Tuesday which was evident in his hysterical phone call with his children.

"He was very emotional, very upset," Farmer's husband Simon told the ABC.

Hannah Clarke was a devoted mum to her three children. File photo / supplied
Hannah Clarke was a devoted mum to her three children. File photo / supplied

"Hannah noticed there was a distinct change... a high level of emotion, he was crying."

The next morning, Baxter doused his family in petrol and set them alight while Clarke was dropping the children off at school.

The three children died at the scene while Clarke succumbed to horrific burns in hospital later that day.

Hannah Clarke with son Trey. Photo / supplied
Hannah Clarke with son Trey. Photo / supplied

Baxter then took his own life.

Nathaniel said his sister's husband had given off red flags in hindsight, but he never believed he would be driven to commit murder.

"Hannah had her suspicions that he might try to do something to her, but we all thought, 'No, he couldn't'," he said.

"It was just a vicious attack to make her suffer as much as he could and that was it. I still can't get over it."