A grieving and angry dad is searching for answers after a government agency placed his 2-year-old son in the care of his grandmother who later brutally murdered him.

The paternal father of Jermain Mason Ngawhau spoke to the Herald exclusively after the toddler's nana, Kathleen Elizabeth Cooper, was jailed for life today.

Jermain's dad, who asked not to be named, talked of his son's funeral just two days before Christmas in 2015.

He remembered the now 66-year-old Cooper singing at the service for the boy lovingly known as Tiger.

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Cooper was at the time lying to family and denied any wrong doing.

"I just want to try block it out, it's just something that will make me upset - yeah it does make me angry," Jermain's father said.

Just a week and half earlier on December 13, Cooper was coming down from a meth-induced high. She was struggling to cope with the pressure of childcare and snapped when Jermain had a toileting accident.

She threw the 10.3kg and 79cm tall toddler down the hallway of their home.

Jermain had also endured learning difficulties and struggled to walk unaided. He suffered what would be fatal head injuries and died five days later on December 18 when his life support was switched off at Starship Hospital.

"I remember seeing him lying there on the operating table - with metal staples along his head," the toddler's dad said.

Cooper attempted to explain Jermain's death with several excuses, including shifting the blame to her 4-year-old granddaughter, whom she claimed hit Jermain over the head with a computer tablet.

Cooper also told police she had phoned for an ambulance immediately after discovering her grandson's injuries, but telephone records show otherwise.

Jermain had been removed from the care of his father and mother by Child Youth and Family (CYF), now the Ministry for Vulnerable Children - Oranga Tamariki, along with his three pre-school siblings, two of which are children of Jermain's dad.

The agency decided to place the children with Cooper after deeming Jermain's mother, Nadia Ngawhau, unfit to care for them, while the toddler's father said he had recently lost his job and was suffering from mental health issues.

"There was nothing I could do - I wasn't able to care for them - I didn't have the money," he said.

Ngawhau told the court at trial that she was "very disappointed", sad and "quite upset" that CYF had taken her children but that they loved their nana. She described her relationship with Cooper as "great".

However, her mother was abusive.

The court heard that Cooper was a "harsh disciplinarian" who smacked the children with her hand and, at times, a belt.

Childcare workers noticed bruising on the children, while a pathologist found bruising on Jermain's body.

A trained butcher, now back working, Jermain's father said the last time he saw Jermain was when he was taken away by CYF.

"CYFs made me feel [like] I was unsafe around him," he said. "Just be straight up with me, you know. Tell me if I can see my kids and maybe have my fatherhood back. It's quite depressing and mentally straining."

He said ultimately he didn't want anyone else to experience the trauma of losing a child in what was supposed to be a safe environment.

"[I've] tried counselling and all that type of stuff ... it's just something I don't want anyone else to go through," he said.

His two daughters, whom he hopes to reform a relationship with, are now fostered to two different families.

"They seem like nice people," he said. "But I always want to see my kids."

A foster parent for one of Jermain's sisters told the court today that the young girl has undergone counselling as a result of her brother's death.

The "beatings and blatant lies" have scarred her, the court heard.

Oranga Tamariki (CYF) said in a statement that Jermain's death was a tragedy, however wouldn't comment further about his placement with Cooper.

"Now the criminal process is completed the coronial process, which could include a coronial hearing, may resume," it said. "We are unable to comment in any detail until that has taken place. The questions raised may be considered at that stage."

Cooper's lawyer, Paul Dacre, QC, said today that CYF must have decided that Cooper was an adequate caregiver for the children.

When sentencing Cooper in the High Court at Auckland, Justice Sarah Katz said Jermain's death has rightly caused public outrage.

"No sentence I pass can bring Jermain back or heal the harm and loss of those that loved him," she said.

Cooper was sentenced to life imprisonment with a minimum term of 14 years and six months.