Tamihana, the court heard at sentencing, had come out of a marriage of 20 years when he met the victim.
On May 30, 2019, he was on bail over a range of domestic-violence allegations (charges that were later dropped) and breached his conditions by meeting the woman.
After a trip to the police station for the woman to withdraw her police complaint, the pair went to Portobello, where Tamihana was staying in a caravan.
An argument that began in the car continued when they were inside and resulted in the defendant pushing the victim on to a bed.
After an abandoned attempt to stab himself in the stomach with a hunting knife, Tamihana swallowed nine prescription painkillers and made the victim take one.
He disabled his girlfriend's cellphone then turned the stove on, releasing the gas he hoped would kill them.
Finally, Tamihana 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 the victim.
However, she waited for Tamihana to pass out before sneaking through the window and alerting the camp manager.
Although the victim spoke to the prisoner over the phone more than 100 times as he awaited sentencing, the board heard that contact had ceased.
Tamihana said he was no longer in a relationship with the woman and had not heard from her since September.
"Mr Tamihana is sincerely remorseful for what he did," panel convener Tania Williams Blyth said.
"He knows he should have dealt with his issues in a better way."
The prisoner was described by staff as "polite, honest, and respectful" but he was also referred to as "a complex person to manage" since he was trying to distance himself from gang connections.
Tamihana will be eligible for parole again in September.
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