UPDATED 11.23am An appeal by the killers of Moko Rangitoheriri to reduce their manslaughter sentences has failed, as a Court of Appeal judge rules their punishment is "entirely within range and not manifestly excessive".

The Court said the pair should count themselves "fortunate" they were not jailed for life.

Tania Shailer and David Haerewa were jailed in June last year for 17 years with a minimum non-parole period of nine years after pleading guilty to the manslaughter of 3-year-old Moko.

They appealed their sentences, saying they were too harsh.

Moko died in August 2015 while in the care of the couple. Shailer was a friend of Moko's mother Nicola Dally-Paki.

Dally-Paki was in Auckland with another child who was sick in Starship Hospital, and asked Shailer to take care of Moko and his older sister while she was away.

In the weeks leading up to his death Moko suffered horrendous and brutal abuse at the hands of the couple.

A post-mortem examination revealed the little boy died as a result of "multiple blunt force traumas", his brain was swollen and he had blood clots under his scalp representing numerous injuries inflicted at different times in the lead-up to his death.

There was also evidence the toddler had been smothered and he had lacerations and hemorrhaging deep within his abdomen, historic bruising and damage to his bowel.

Combined, that resulted in his bowel rupturing. Faecal matter leaked into Moko's abdomen, causing septic shock.

Shailer and Haerewa pleaded guilty to abusing Moko and received the longest sentence handed down in New Zealand for the manslaughter of a child.

Justice Stephen Kos said the crux of the appeal was whether the offending was "within the most serious of cases of manslaughter".

"That question arises both generally and specifically in relation to Ms Shailer who was otherwise of good character, but suffers some mental health disorders," he said.

"Do those disorders mean that her level of culpability is reduced, so that in her case at least this is not within the most serious of cases of manslaughter, and so alter the starting point for her sentence?

"And in the case of Mr Haerewa, is his offending less serious than Ms Shailer's, so that his offending too is not within the most serious of cases of manslaughter?

"These questions concerning the sentence starting point are the primary questions in this appeal."

The maximum penalty for manslaughter is life imprisonment and sentencing judge Justice Katz adopted that as a starting point.

However, she concluded, after giving the Shailer and Haerewa discounts for their guilty pleas and personal mitigating circumstances, a life sentence was not appropriate.

"But were the discounts she gave adequate? And was the minimum period of imprisonment too high? These questions concerning the sentence end point are the secondary issues in this appeal," Justice Kos continued.

The first specific point of the appeal was whether Shailer's mental health disorders "materially diminish her culpability for the offending".

Justice Kos said Shailer's mental health problems "may have contributed significantly to the most serious event within the whole course of conduct constituting the offending".

"But that is one element only among the events causing Moko's death for which she is culpable," he said.

"It cannot seriously be said to be causative of the other major elements: Moko's head injuries and the failure to obtain medical assistance."

Justice Kos ruled that Shailer's mental health disorders "did not materially diminish her culpability for her offending and therefore do not reduce the starting point of her sentence".

The second point was whether Haerewa's participation materially diminished his culpability.

Haerewa claimed Shailer was the main instigator of the violence towards Moko and he was merely a "secondary party" and Justice Katz should have acknowledged that.

However, Justice Kos said Justice Katz was right not to draw a distinction between the killers.

"By no means could Mr Haerewa's involvement be described as "secondary"," Justice Kos said.

"The summary of facts, accepted by both the appellants, identifies three fundamental causes of death.

"One was the stomping, and for that Ms Shailer was primarily responsible, subject to the qualifications identified by the Judge. Secondly, the head injury to Moko that caused brain swelling. For that Mr Haerewa must take every bit as much responsibility as Ms Shailer. Thirdly, the failure to get Moko medical attention. The same observation applies."

The Court of Appeal therefore rejected Haerewa's argument.

The third point of appeal was whether the offending was "within the most serious of cases" of manslaughter.

The couple were initially charged with murder but that was later reduced and they pleaded guilty to manslaguther.

Given it was impossible to tell which injury killed Moko, a murder charge would have been extremely hard for the Crown to prove.

Justice Kos pointed out that Moko was beaten persistently, and bitten on a number of occasions.

His death was the result of "a joint enterprise perpetrated over a period of weeks leading up to the killing of a defenceless child by a caregiver".

Justice Kos said the case "had to be viewed by the Courts at least as (and probably more) seriously than a single extreme assault on an adult".

"This case is among the most serious cases of manslaughter," he ruled.

The fifth appeal point was whether sentence discounts for Shailer and Haerewa's personal mitigating circumstances were adequate.

That point also failed: Justice Kos ruled that neither sentence could "conceivably be described as manifestly excessive".

Justice Kos also dismissed the final point of the appeal - whether the minimum period of imprisonment too high - ruling "we are not persuaded that the sentencing Judge erred".

"The duration, cruelty and physical depravity of this offending by two adults acting together against a defenceless 3-year-old child placed it within the most serious of cases for which the penalty of life imprisonment is prescribed.

"All the recognised sentencing principles called for a term of imprisonment which reflected society's denunciation.

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

- NZ Herald