The Crown wanted to secure a conviction over the death of toddler Moko Rangitoheriri rather than risk a jury trial for murder where they could escape penalty, the Attorney-General says.
Tania Shailer and David Haerewa were originally charged with murder but were able to accept a plea deal for manslaughter, which angered many.
Protests were held outside court houses across New Zealand on Monday leading up to the sentencing in the Rotorua High Court.
Shailer and Haerewa were each sentenced to 17 years' prison, with a minimum nine year non-parole period.
Three-year-old Moko was subjected to days of abuse by the pair who were meant to be looking after him, including being stomped, beaten and locked in the bathroom for extended periods of time.
His mother, Nicola Dally-Paki, said she couldn't recognise her son when she went to identify him in the morgue.
In a rare move, the Attorney-General Chris Finlayson has sought to explain why the plea bargain was struck.
"The Crown's decisions in this case, including the decision to accept the manslaughter pleas, were motivated by the need to secure convictions for this horrendous killing and to avoid the significant risk that either of the defendants could escape such a conviction because of evidential issues," he says.
"The guilty pleas and admitted facts enabled the Crown to argue for a sentence which reflected the nature of the crimes committed. Without the guilty pleas, the full details of the facts set out in the statement of facts may not have otherwise come to light."
Mr Finlayson says any decision to substitute murder with manslaughter is "never taken lightly". The process ultimately required the approval of Solicitor-General Una Jagose.
Based on evidence which would have been available at trial, Mr Finlayson says there was "substantial risk" one or both of the pair wouldn't have been convicted of murder or manslaughter.
"To prove the legal charge of murder in this case, the Crown was required to prove beyond a reasonable doubt that Moko's fatal injuries were inflicted with murderous intent."
The Crown was confident Shailer was the lead offender who was most likely directly responsible for Moko's death.
"The nature of Mr Haerewa's involvement in Moko's fatal injuries was less clear on the evidence available for trial," Mr Finlayson says.
"It was relevant to the likelihood of securing a murder conviction that the injuries Moko suffered were not inevitably fatal. With reasonably prompt medical treatment, he could have been saved."
However, Mr Finlayson said offering the lesser charge "in no way reduces the seriousness of the abuse Moko suffered".
"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.
"Of course, the defendants also had a responsibility to obtain the medical treatment which would have saved Moko's life. By failing to discharge this duty, they contributed to his death in addition to causing his fatal injuries."
Mr Finlayson says the cost of prosecution was not taken into account when offering the manslaughter charge, and the guilty pleas have saved witnesses, including children, from having to give evidence.
Prime Minister John Key didn't want to comment on the sentencing at his post-Cabinet news conference, but called the case "tragic".
"What you are seeing from New Zealanders is that they are responding to not only this particular case, but violence and abuse against children, and rightfully so. They are demanding a higher level of protection and care for these young people."
He says that's why the Government is looking at putting in place a major reform of Child, Youth and Family.
Mr Key says Crown Law is the right body to make the call on which charges should be laid.
"The important point is they will always make a decision based on what they think will be successful."
"If in their judgement they were more likely to have a successful prosecution for manslaughter versus the risk they might be unsuccessful in whole or in part for a murder charge then you'd probably say they've taken the right step."
The New Zealand Bar Association says it has "every confidence" in the Solicitor-General's process in coming to the decision.
Newshub.
