Whanganui man Scott Millar doused a house with petrol intending to kill his former partner, but in the process also killed their baby and himself.
The coroner has just released findings into the triple fatality on March 17 last year, which reveals for the first time a history of domestic violence.
The finding details how Millar had previously served jail time for smearing petrol on a former partner's chest and face and threatening her.
Millar had a four-month-old baby, Ash, with his then partner Katie Bruce.
The mother and son's badly burnt bodies were found inside the house on State Highway 5 at Rangitaiki about 3am on the morning of the fire. Their bodies were indistinguishable, the coroner's finding said.
Millar suffered serious burns and was taken to Waikato Hospital where he died four hours later.
Coroner Wallace Bain has ruled Millar, a forestry worker, intended to kill Bruce and, while Millar was reckless by pouring petrol inside the house and lighting the fire near Ash, he didn't intend to kill the baby or himself.
Coroner Bain's finding said Millar went from Whanganui to the house on an irregular basis to visit his baby. He said there were "ongoing domestic violence issues".
Included as background, Coroner Bain's finding said Millar had been in a relationship with another person for a year and they had a five-month-old daughter together.
That relationship ended due to Millar's verbal abuse and he was trespassed from that woman's house.
In December 2009, he threatened to burn down that house and the people inside. He had a full bottle of petrol and smeared it on the face and chest of his former partner.
On the night of the Rangitaiki fire, Millar was found outside the house by police attending the blaze.
Ms Bruce's body was found near a pull-out bed and Ash's body was found beneath the metal framing of the pull-out.
There were other people at the house who survived the fire, including Bruce's sister.
Millar's mother from Whanganui, who did not want to be named, told NZME the situation was "shocking" and an "absolutely tragedy" for both families.
"That's not who we knew ... he was still my son and I still loved him. We feel absolutely dreadful but there's nothing we can do to change it — we can't bring them back."
The mum said her son and Ms Bruce were often at "loggerheads"but no one knew their relationship would end like this.
"I have lost my son and my grandchild."
A Bruce family spokeswoman said they were not up to speaking to the media. They were focusing on getting through Ms Bruce and Ash's one year memorial on Saturday.
Coroner Bain has issued his finding without holding an inquest
If it is an emergency and you or someone you know is at risk, call 111.
• Women's Refuge:
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(09) 522 2999
• Family Violence Info Line:
0800 456 450