Dylyn Mitchell Davis murdered his girlfriend as a third-strike offence, but avoided a sentence of life imprisonment without parole. (File photo)

A third-strike murderer who avoided life imprisonment without parole, now says even the 20-year minimum term was too long.

Dylyn Mitchell Davis appealed against the sentence that he serve at least 20 years before he can be paroled for the murder of his girlfriend, Aroha Kerehoma, 28, in Hamilton in February.

At a hearing in Wellington on Thursday the Court of Appeal reserved its decision, but Davis' lawyer, Thomas Sutcliffe, faced a string of questions from the three judges about why the 20-year minimum on the life imprisonment term was wrong given the intent behind the "three strikes" law.

FACEBOOK Aroha Kerehoma had texted a former partner saying she didn't feel safe around her new boyfriend Dylyn Davis, which enraged Davis.

At the start of his submissions Sutcliffe acknowledged that with the sentencing judge's comment that it was a finely balanced decision to avoid life without parole, it could be suggested that the 20-year minimum was unlikely to be unjust.

"You are right to appreciate that risk," Justice Forrie Miller said.

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MARK TAYLOR/STUFF Aroha Kerehoma's friends and family turned up in force to Dylyn Davis' sentencing hearing, wearing t-shirts with her photograph and the words "Justice for Aroha". (File photo)

The third-strike law calls for life without parole for murder as a third "strike" unless that sentence would be manifestly unjust. If no parole was unjust, the next step was to impose at least 20 years without parole unless that was clearly unjust.

There was room for the view that Davis was treated leniently, the judge said.

Sutcliffe asked for the non-parole period to be dropped to 14 years. Davis' two previous strikes – for aggravated robbery and wounding with intent to injure – had been nasty but not particularly serious, he said.

Another of the judges, Justice Raynor Asher, took issue with that, saying Davis had the record of a "highly dangerous man".

The Crown's lawyer, Charlotte Brook, said a 14-year minimum would have been far too low. Davis was 26 when he was sentenced, so life imprisonment without parole could have meant 50 or 60 years in jail.

It was only that factor that stopped the Crown from saying at the appeal hearing that Davis should have got life without parole.

Had he been an older man he could well have been the first person sentenced to life without parole under the three strikes regime, Brook said. His risk was so great that the community should be protected from him.

Davis pleaded guilty to the murder charge.

He had beaten Kerehoma to death in the Hamilton garage where they had been living, after she texted a former partner saying Davis had been hitting her and she felt unsafe.

He took her phone from her and read the texts. His anger turned to violence and it appeared he punched her to death, breaking bones in her face and the back of her skull, and probably kicking her too.

He had killed Kerehoma just five months after being released from prison. He left her to die in the garage, went to see friends and only called an ambulance when he returned hours later, when she was long dead.