Attorney-General Christopher Finlayson said that accepting the manslaughter pleas and admitted facts meant prosecutors could push for a sentence that reflected what had happened to Moko.
"Without the guilty pleas, the full details ... may not have otherwise come to light.
"Based on the evidence available for trial, there was a substantial risk that one or both of the defendants would not be convicted of the legal charge of murder or manslaughter."
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To prove murder, it would need to be proven beyond a reasonable doubt that Moko's fatal facial injuries were inflicted with murderous intent.
"The injuries Moko suffered were not inevitably fatal. With reasonably prompt medical treatment, he could have been saved.
"If the jury was not satisfied beyond a reasonable doubt that Ms Shailer had murderous intent at the time she inflicted the fatal injuries, then neither she nor Mr Haerewa could have been convicted of murder."
The defendants had a responsibility to get medical treatment for Moko, Mr Finlayson said. By failing to do so, they contributed to his death, in addition to causing his injuries.
"But the failure to discharge this duty could not in itself lead to a murder conviction for both defendants nor would a conviction for manslaughter based solely on this failure have adequately reflected the defendants' role in inflicting the injuries which killed Moko."
Labour's shadow Attorney-General David Parker said it should have been left to a jury to decide if Shailer and Haerewa were murderers.
If that happened, Mr Parker didn't accept a significant risk of no conviction.
"When they kicked this boy so hard that he flew across the room and landed in the hall in the next room, when they stomped on his stomach repeatedly, they had to prove that when they did that these people knew it was likely to cause Moko's death.
"That is a question of fact that should have been left to the jury. These people are liars, we know they are liars."
New Zealand First leader Winston Peters said the downgraded manslaughter conviction reflected a trend of "political systems disguising the real nature of crime".
"This was a murder of the very worst sort and the charges should never have been downgraded."
The New Zealand Bar Association also defended the process of downgrading murder charges to manslaughter, saying it has "every confidence" in the Solicitor-General's considerations.
It's also hit out against misinformed pubic debate from those who don't have all the evidence about cases at their fingertips.
"Suggestions, reported in the media, that the process is flawed or that financial implications are relevant to the decision are misconceived and wrong," said association president Clive Elliott, QC.
"Unlike the public, the solicitor-general will have carefully considered all the available and admissible evidence in determining the appropriate charge.
"The tragic cases involving the death of a baby or an infant often present evidential challenges for the prosecution. If the admissible evidence does not meet the threshold to secure a conviction on a particular charge it will be necessary to down-grade the charge to one where the threshold is satisfied."
The association "fully supports" public debate on such issues, particular to ensure the public properly understood the processes involved, Mr Elliott said.
"That debate is one which will be more appropriately engaged following the determination of the present Baby Moko case. It is only then that the full facts can be released to ensure the debate is adequately informed."
The New Zealand Law Society has also come out in defence of the "robust" guidelines Crown prosecutors follow when considering "so-called plea bargains".
In this case, the prosecutors relied on the 2013 guidelines that say discussions and arrangements about pleas "have a significant value for the administration of the criminal justice system".
That value included "relieving victims or complainants of the burden of the trial process", something Law Society president Kathryn Beck said was relevant when vulnerable victims or witnesses were involved.
"When making any decision as to charges all prosecutors are bound by the guidelines, which were thoroughly reviewed in 2013 and have been developed over a long time with the benefit of years of experience in practice," she said.
"Importantly in the present context, decisions by prosecutors to accept a lesser charge than murder in relation to a homicide case are made in consultation with the deceased's family. Such decisions must be approved by the Solicitor-General through the Crown Law Office."
Ms Beck said the Law Society believed New Zealand had "a robust process and clear guidelines" for Crown solicitors to make these calls.
"While never easy decisions to make, the Law Society is satisfied that the process that must be followed by decision-makers is fair, robust, and in accordance with the interests of justice and the public," Ms Beck said.
"In relation to today's sentencing, the charge of manslaughter still allowed the judge to impose a lengthy period of imprisonment - reflective of the serious nature of the offenders' crimes - as manslaughter carries the same maximum penalty as a murder charge: life imprisonment."
Police could not comment on the specifics of the prosecution as the Coroner has indicated an inquest into Moko's death will be held.
Sensible Sentencing Trust founder Garth McVicar said the sentence showed Justice Sarah Katz had "gone as far as possible under the current legislation".
"The police originally charged [Moko's killers] with murder ... those charges should have stuck and the sentence would've been life without parole."
Justice Minister Amy Adams had called for the public to stand up for vulnerable children in the community and today, in protests around the country, they had, Mr McVicar said.
It was now time for lawmakers to step up.
"For New Zealand to get on top of this horrific level of child abuse we need to redefine the boundaries and redefine the consequences for anybody who hurts or abuses a child ... we were surprised [Justice Katz] went the whole way in sentencing. It shows her frustration."
- additional reporting Jimmy Ellingham