The killing of Taupo toddler Moko Rangitoheriri should have gone to trial despite the impact on his family, a law expert says.

The toddler's two carers, Tania Shailer and David Haerewa, had a murder charge downgraded to manslaughter, prompting calls for tougher rules governing plea bargaining and an overhaul of the homicide laws.

Court documents show Moko Rangitoheriri suffered severe injuries, inflicted over a long time, while being looked after by the pair, who admitted killing him.

Photo: NZ POLICE

Massey University's College of Humanities and Social Sciences deputy pro vice chancellor Chris Gallavin said police should have pursued the murder charge.

"It's not a case that this was temporary insanity on behalf of the police and they've just gone ahead and wanted to improve the statistics, there will have been consideration of what was in the wider interests of justice.

"I personally think that actually the importance of a conviction for murder and also what we know as far as has been released to the public, and we don't know all the circumstances, but I think that they had a very good chance of securing a conviction for murder and I think in the context of the severity and the public concern that they ought not have agreed to a plea to manslaughter and they should have gone for the murder charge."

There were two reasons the police would have downgraded the charge to manslaughter, he said.

"One is, and I'm sorry to put it into such crass terms, but a bird in the hand is worth two in the bush, as far as securing a conviction that may not have been guaranteed if they went ahead with the charge of murder.

"Second, undoubtedly there would have been some consideration of the interests of the victim's family and to have that full confession with some light being shed upon what actually happened in the circumstances can help inexorably with the healing process and with the process of recovery for the family."

A murder trial would have taken months if not a year and the true story of what happened may never have actually come out because there was no incentive for the defendants to come clean, Professor Gallavin said.

He wanted more oversight of plea bargains to ensure the deals did not happen unchecked and police did not take advantage of the defence.

"I think that the balance has gone more towards dealing with the nuances of such cases at sentence, and that more focus should be on the conviction and prosecution stage."

All varieties of homicide cases came under one headline of murder, which meant prosecutors had no real guidance for fitting a specific charge to the type of offence that had happened, he said.

"That's what can give rise to perversities of plea bargains, it can give rise to perversities of people being overcharged with murder when it might have been a shop keeper defending himself in the context of an armed intruder."

The problem could be tackled by having different degrees of murder, he said.

Sensible Sentencing Trust's Garth McVicar agreed.

He told Morning Report prosecutors were encouraged by law to get rid of a case as soon as possible, for financial reasons.

"What has happened here is the prosecutor has negotiated a guilty plea of a lot lesser charge obviously, of murder to manslaughter, so the country isn't faced with the costs and ultimately the prosecutor has achieved what he has been dictated to by legislation."

Saving money should not be the aim in child abuse cases, he said.

Photo: SUPPLIED

"I mean it has saved the country money and if we are all about saving money then that has been achieved, but we are not about that in my opinion. We should be trying to redefine the boundaries around child abuse."

Photo: Supplied

If abuse like this case suddenly becomes manslaughter, it lowers the bar - when the penalty should be tougher, Mr McVicar said. The trust wants an end to such plea bargains.

But Te Whanau o Waipareira chief executive John Tamihere said that's a middle class response to a lower class problem, and toughening up laws won't mean people who commit such inhumane acts are even aware of the penalty.

There has to be earlier and more co-ordinated intervention - with resources put into stopping "the intergenerational spiral into insanity that sees violence perpetrated against women and children", Mr Tamihere said.