Labour children's spokesperson Jacinda Ardern has slammed the Crown Prosecution Service for making a deal which saw the killers of Moko Rangitoheriri plead down to manslaughter.

Jacinda's comments come out the day after Moko's mother, Nicola Dally-Paki, spoke publicly for the first time about her son's death.

Moko Rangitoheriri.

It's not hard to see the toll the death of her son has taken. It's tattooed plainly on her face for everyone to see. On her right cheek written in Spanish 'see no evil, hear no evil, speak no evil”.

Above her left eye her daughter's name and below the left eye a red diamond and a tattoo across her neck representing the son she lost.

Nicola Dally-Paki. Photos: Stuff.

Nicola entrusted David Haerewa and Tania Shailer with the care of her son and her eight-year-old daughter in Taupo. She trusted them to give her children the best care while Nicola was looking after another one of her children in Starship Hospital.

However, the duo violently abused Moko for weeks, kicking, slapping, stomping on him and rubbing faeces in his face before his death on August 10, 2015.

The harrowing story was televised on Newshub's Story last night.

Nicola says she should have picked up on the signs. One particular incident which haunts the mother was a conversation she had with Shailer before her three-year-old son's death.

'Your f***ing son smashed a window, I'm so mad right now,” Shailer told Nicola.

Nicola says she offered to pay for the broken window and asked if she could talk to her son.

But Shailer told Nicola, she (Shailer) needed to calm down and Moko had gone to bed and to call back later.

It was the next day before she was able to get in contact Shailer and talk to her son.

'He was quiet, and she just said that because it was near bed time that he was just tired. Those are signs, those should have been the signs that I should've picked up on.”

Her daughter only told her what had happened once the child knew both Shailer and Haerewa were in jail.

The daughter would use toilet paper to wipe Moko's bleeding eyes. Shailer had taped, and drag him to the wardrobe to hide him from them.

"She said she was told to tell the police that she had hurt Moko. 'If you don't, I will do to your mother what I did to Moko'," Nicola told Story.

"They psychologically screwed with her head."

In the autopsy it was ruled it was difficult to determine the exact cause of death due to the number of injuries Moko received, it was reported.

Lacerations and a haemorrhage deep within Moko's abdomen, coupled with a bowel rupture - likely caused by being stomped on was one potential cause.

The other cause of death could have been swelling to the brain from repeated blows to the head.

Moko's injuries included 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 and making him very weak.

The pair pleaded guilty to manslaughter charges.

Jacinda says the manslaughter plea was outrageous and shouldn't have been offered by the Crown Prosecution Service.

'I have looked over 40 cases of child abuse cases, and while manslaughter is a charge that features, it's unfathomable that the Crown Prosecutors used it in this case.

'Shailer and Haerewa were originally charged with murder in October last year, only to have that downgraded to manslaughter – a charge they then pled guilty to in May.

'Our law was not the problem here. The plea deal was.

'Coroner Bain, who will conduct an inquest into this case, has drawn comparisons with Nia Glassie. Moko was the victim of a sustained attack, and what has been described as torture," says Jacinda.

'This case was horrific, and it is hard to see how this could have been interpreted as anything other than murder.

'Justice would have been better served by allowing this case to go to trial, and allowing a jury to decide,” says Jacinda.

- additional reporting Stuff.co.nz