A man who at 14 was one of New Zealand's youngest killers has been jailed again for a vicious attack on his pregnant partner - and has been warned if his violence continues he could spend his life behind bars.

Anthony Alfred Afu, 28, punched his partner, who was five months pregnant, hard in the stomach as he shouted "you don't deserve to have my baby".

The December 2007 attack lasted several minutes until his partner, who cannot be named, got away from him and fled inside a house. Afu forced his way in, dragged her outside, and continued beating her.

In 1994 he became one of New Zealand's youngest killers when, just after he turned 14, he attacked John Wahanui with a piece of wood after a drinking session. When Mr Wahanui died, Afu was jailed for two years three months for manslaughter.

Afu's partner was left with extensive bruising and swelling to her stomach. Four days later tests showed her baby was dead.

Afu was initially charged with procuring an abortion but the Crown couldn't prove the assault caused the baby's death.

He eventually admitted injuring with intent to cause grievous bodily harm, male assaults female and injuring with intent to injure in March before he went on trial.

The charges related to the assault on the partner and another incident where he knocked out two of her teeth and beat her 3-year-old son so badly he was hospitalised.

A victim impact statement from the partner couldn't be obtained but after the attack she told police she mourned for her unborn baby and her son who had been taken from her care.

In the High Court at Auckland yesterday, Justice Chris Allan urged Afu to use his time in jail to "take stock of your life" and to "make good on the letter you wrote to the court".

Afu's letter said he was mortified by his actions and was sorry for the pain he'd caused.

Justice Allan said Afu had been put on notice that if he continued to offend like he had throughout his life the Crown would seek a sentence of preventive detention, which is an indefinite sentence.

After he had been released on the manslaughter charge, he was jailed twice more for assaulting females.

Afu was sentenced to six years and nine months jail with a minimum non-parole period of four years.

Act MP David Garrett, whose "three strikes" bill is about to go to a Parliament select committee, told the Herald he would need to look at Afu's criminal history to see if the legislation could be applied to him.

However, he said the problem with preventive detention was that it wasn't used by judges enough.

"It's effectively giving the guy another chance to kill."