Shouts of warning from a teenager about to be brutally murdered by his half-brother almost certainly saved the life of his younger sister, a court has heard.
Nathan Gordon Frost was jailed for life with a minimum parole period of 20 years when he appeared in the High Court in New Plymouth for sentencing today.
The 21-year-old used a pipe wrench and 12cm hunting knife to kill his father Stephen John Frost, 55, and Regan Frost-Lawn, 15, at a Hawera address in the early hours of 18 January.
Nathan Frost held a deep resentment over his parents' separation when he was young, which had only intensified after his mother died six years ago.
Justice Rebecca Ellis said on the night of the murders he had consumed a bottle of whiskey and attacked his father after he came to check on him because he had heard him crying.
"In your room was a large pipe wrench you had taken there at some earlier time with murder in mind. You struck your father multiple times in the head and arm.
"You broke his jaw in two places and knocked him unconscious as you continued to strike him with the wrench.
"You then unfolded a hunting knife with a 12cm blade that you had bought not long before. You stabbed Stephen in the neck fatally severing his carotid artery and jugular. Stephen died at the scene."
Justice Ellis said Regan Frost-Lawn had heard and witnessed part of the attack and yelled at his brother to stop and to warn his sister Grace that Nathan had a knife.
"You then went into Regan's bedroom and stabbed him many times in the back and shoulder puncturing his lungs and causing him massive internal bleeding. You then made a fatal attack on his head and neck.
"You then got the keys to the sleepout, but fortunately Grace had heard Regan yelling and had escaped. She managed to hide nearby and called the police. I have no doubt Regan saved Grace's life that night."
Grace Lawn fought back tears as she read her victim impact statement.
"I've had sleepless nights wondering why someone has such a vendetta against their own brother and father, especially after all dad has done for you to try and get your life back on track and stop you sitting on the couch drinking your life away.
"You took our only father, who had ripped his life apart to make sure us kids had food in our mouths."
Grace told the court she required counselling and was suffering anxiety.
"I have days when I just sit and blame myself for what happened, but on reflection I count myself lucky that I left my sleepout when I did because I feel as though that if I didn't leave when I did, you would have killed me too. But just not in a heartbeat I would swap those two out for me."
Grace told her brother that she still loved him.
"Nathan, I care about you. You had the potential to go far in life, but what you have done makes me sick with unbearable pain and hurt. I still love you and want what's best for you whatever that might look like in the future."
Stephen Frost's sister Alison Turner said she had put her anger to one side.
"I chose to feel sorry for you. You are a pitiful, self-centred nobody. A stronger person would've acknowledged they needed help. You chose not to. You acknowledge nothing, instead you took two lives."
Another sister Sharon Wimsett was not so forgiving.
"I hate what you've done to our family and I'm so angry with you for causing us so much grief and forever having to know what you did to them.
"I hate that they died violently and brutally and that Regan must have been terrified and I pray that Stephen didn't understand what was happening."
Wimsett said the family stood united in their grief, while Nathan Frost was alone and she hoped he would remain that way the rest of his life.
Stephen's brother Grant Frost told Nathan he would never forget or forgive him for the cowardly attack on his brother and nephew.
"There is no place on this Earth for a disturbed soul like your's, who kills their own kind in cold blood. Your horrific actions have affected and changed our family forever. A family you will never be involved with or be a part of ever again.
"If there is any justice in this world I hope you never get parole and you spend the rest of your breathing life incarcerated."
In sentencing, Justice Ellis said, although the psychiatrists found that Nathan Frost was fit to plead and sane at the time of the murders, it was clear he had mental health problems.
But she said that was not an excuse for and did not fully explain his offending.
Justice Ellis said Nathan Frost struggled to express remorse or guilt for the murders and it was hoped a mental health diagnoses would reveal itself over time if he engaged in treatment.