The defendant apologised only for his "part in the case" and attributed his actions to the psychological hardships each of them were experiencing.
"But it was you, Tamihana, who sought to murder the other person, not the other way round," the judge said.
Tamihana met the victim while they were each undergoing treatment for respective mental-health issues, the court heard.
It had only been months since the break-up of his 20-year marriage and the issuing of a protection order keeping him away from his wife and children.
"It's hard to imagine a man more vulnerable," counsel Anne Stevens QC said.
The commencement of the new relationship with the victim, she said, was part of "a terrible formula" which had a near fatal conclusion on May 30 last year.
On bail for domestic-violence charges — which were later dropped — Tamihana breached that bail by meeting his victim in central Dunedin.
After a trip to the police station for the woman to withdraw her police complaint, the pair then drove to the Portobello Village Tourist Park where the man was staying.
An argument began en route and became physical once they were inside the caravan.
They argued and Tamihana pushed the victim on to a bed.
She was not leaving, the defendant said.
After an abandoned attempt to stab himself in the stomach with a hunting knife, Tamihana downed nine prescription painkillers.
He made the victim take one and then began his bid to end their lives.
First Tamihana removed the woman's cellphone battery and sim card, then turned the stove on, releasing the gas he hoped would kill them.
Finally, he closed the vents and parked his car beside the caravan's door, climbing back in through the bathroom window.
"We are going to die together," he told his girlfriend.
The couple had no idea, but later analysis showed it would have been virtually impossible for them to die from gas inhalation.
The victim deliberately took shallow breaths and waited for Tamihana to become unconscious before making her bid for freedom. Her first attempt was thwarted when the man grabbed her leg but eventually he succumbed to the medication and she fled through the window, alerting the camp manager.
When first interviewed by police, the victim said she was haunted by the ordeal.
"My biggest fear is that when Blair is eventually released he will hunt me down and finish the job," she said.
"I am absolutely certain that he will do everything in his power to find me and I am petrified for my life."
But absence, it seems, softened her stance.
When Tamihana was behind bars awaiting sentence, the woman set up a fake email address to communicate with him and spoke with him over the phone more than 100 times.
"I started looking at old photos of Blair and me and remembering all the good times we had, there were lots of them," she wrote in an updated statement last month.
It was good to hear Tamihana apologise for his actions, the victim said.
"I'm willing to give him one more chance at a relationship but it must be without drugs in his life."
Stevens her client believed he was executing a suicide pact they had both agreed on and argued there was a lack of malice.
Justice Osborne said that was not backed up by the man's attempts to prevent escape.
"It's clear ... your contact towards the victim in the caravan was fuelled by a sense of grievance, if not outright anger, at some unclear aspect of her behaviour," he said.
He issued a protection order in favour of the victim.
• 0800 543 354 (0800 LIFELINE) or free text 4357 (HELP) (available 24/7)
• https://www.lifeline.org.nz/services/suicide-crisis-helpline
• YOUTHLINE: 0800 376 633
• NEED TO TALK? Free call or text 1737 (available 24/7)
• KIDSLINE: 0800 543 754 (available 24/7)
• WHATSUP: 0800 942 8787 (1pm to 11pm)
• DEPRESSION HELPLINE: 0800 111 757 or TEXT 4202
• NATIONAL ANXIETY 24 HR HELPLINE: 0800 269 4389
