Moko suffered abuse at the hands of caregiver Tania Shailer and her boyfriend David Haerewa.

The Court of Appeal has made clear that being charged with murder or manslaughter can mean life - if the crime fits.

The court's rejection of Moko Rangitoheriri's killers' bid to have their sentence reduced was released on Friday.

The three-year-old's death shocked the nation - shock that turned to anger and revulsion when details emerged of the abuse the toddler suffered before his death in August 2015.

GEORGE HEARD/FAIRFAX NZ Tania Shailer and David Haerewa were each handed 17-year prison sentences for the manslaughter of Moko Rangitoheriri.

David Haerewa and Tania Shailer were initially charged with murder before pleading guilty to the lesser charge of manslaughter.

They were each sentenced to 17 years in prison with a non-parole period of nine years - the longest manslaughter sentence in New Zealand history.

And now they must serve it.

The decision released on Friday rejected five issues raised by Moko's killers which they believed were not considered when they were sentenced by Justice Sarah Katz in June last year.

Criminal defence lawyer Steve Bonnar, QC, said the decision makes clear that life sentences can be imposed on manslaughter charges.

"I suppose it emphasises that the sentencing outcome is not necessarily different, because you can be looking at the same sort of sentence for manslaughter as you might get for murder," he said.

Bonnar, who is also the New Zealand Law Society's criminal law committee convenor, said all Court of Appeal judgments create a precedent, but the rejection of Shailer and Haerewa's appeals reinforced the law.

"What the Court of Appeal has acknowledged is exactly right, that obviously manslaughter carries life imprisonment, as murder does," he said.

"If you get a case which is in the most serious category of manslaughter, which the Court of Appeal said this was, then you can take a starting point of life imprisonment for manslaughter.

"The distinction between murder and manslaughter is still important, but this case just makes it clear that in the worst sorts of manslaughter, sentencing is not your worry."

Arama Ngapo-Lipscombe, the lawyer representing Moko's mother, Nicola Dally-Paki, said the latest ruling has set a precedent.

"The Court of Appeal has sent a strong, unanimous message that this type of offending will not be tolerated."

Ngapo-Lipscombe said that the toddler's mother was relieved that her son's killers have had their bid rejected.

"But there is no happy ending here. She will continue to live a life sentence. She will never hold her baby again."

Moko and his sister were left with Shailer while their mother tended to their brother, who was in Starship hospital with an infected leg.

Over two months, the toddler endured daily beatings at the hands of Shailer and, later, Shailer and Haerewa.

When Moko was finally brought to Taupo Hospital by paramedics, he had facial and neck contusions and abrasions; human bite marks on the left cheek, right cheek and arms; lacerations to his chin, neck, ears and lip; haemorrhages to both eyes; and multiple abrasions and contusions over the skin of his chest and abdomen. His bowel was also ruptured, causing infections, making him very weak.

He died in August 2015.

Legal teams representing Haerewa and Shailer raised five issues in the appeal hearing held in November last year.

Shailer's lawyer Ron Mansfield argued that Shailer's offending was a direct result of her mental health disorders, which diminished her culpability.

Lawyer Harry Edwards who represented Haerewa argued that while his client was involved in the offending, he was not directly responsible for Moko's death.

The court was also asked to consider whether the case was the most serious of cases, whether the discount they received for their guilty plea was adequate and whether the minimum period of imprisonment was too high.

The Court of Appeal rejected the arguments and dismissed the appeals.

"The duration, cruelty and physical depravity of this offending by two adults acting together against a defenceless three-year-old child placed it within the most serious of cases for which the penalty of life imprisonment is prescribed," the decision said.

"Ms Shailer and Mr Haerewa can count themselves fortunate that they were not sentenced to life imprisonment."

* An earlier version of this story incorrectly identified Steve Bonnar as a crown prosecutor.