31/07/2017 7:01 PM AEST | Updated 31/07/2017 7:07 PM AEST

Grandma Accused Of Killing Grandson, Blaming It On His 4-Year-Old Sister

The New Zealand grandmother had allegedly been using methamphetamine when she assaulted the two-year-old child.

A New Zealand grandmother accused of the alleged murder of her two-year-old grandson has shifted the blame for his death to her four-year-old granddaughter and claimed she had "no murderous intent" towards the child.

In a New Zealand High Court on Monday, a lawyer for Auckland grandmother Kathleen Elizabeth Cooper, 65, claimed she was responsible for the death of her grandson, Jermaine Mason Ngawhau, but on the grounds of manslaughter and not murder.

Cooper is accused of the alleged murder of the young boy after she assaulted him, resulting in head injuries so severe experts said they appeared similar to those of someone in a high-speed car accident. Despite this, the grandmother has said she believes her four-year-old granddaughter is responsible for the death after hitting Jermaine over the head with an iPad.

In Cooper's defence, counsel Paul Dacre said the lack of "murderous intent" shown by her at the time of the alleged assault provides grounds for a manslaughter charge and not murder.

"It's a tragedy that a young boy hasn't fulfilled his potential, he's been taken away from his family and he's been taken away from the community," he said in his opening statement on Monday.

"Mrs Cooper accepts that she caused the death and that the death was caused by an assault.

"You've got to look at the circumstances and look at what was going through her mind."

New Zealand crown prosecutor Aaron Perkins told the jury the injuries sustained by Jermaine as a result of the alleged assault on December 13, 2015 -- where Cooper is accused of throwing the child down a hallway -- required immediate surgery and left the entire left-half of his brain "effectively dead".

Jermaine died on December 18, 2015 after his life support was switched off.

"He had a severe head injury, emergency surgery was performed at Starship [Hospital], but there was no hope -- the injuries were too severe," Perkins said.

"The damage had been done -- the entire left side of his brain was effectively dead already."

Perkins told the court on Monday that Cooper had been caring for her daughter's four children at the time of the alleged assault and that the grandmother's discipline of them would often include smacking, yelling and slamming doors, with daycare workers citing "quite regular and sometimes quite significant bruising on the children", according to a NZ Herald report.

The prosecutor also claimed Cooper may have been provoked by the developmental issues Jermaine suffered -- he weighed just 10.3kg and had a height of 79cm -- resulting in the alleged assault.

"The Crown suggests that Cathleen Cooper was struggling to come to terms with Jermaine's difficulties... a lack of understanding or sympathy for his position," he told the jury.

Further evidence also showed that forensic tests of hair samples taken from Jermaine and the other children suggested Cooper had been using methamphetamine while in her house.

"Police organised for hair samples to be taken not only from Jermaine but from the other three children," Perkins said.

"This may well have contributed to the frame of mind that [Cooper] was in -- her inability to make allowances for his developmental problems and to cope with those."

The trial is expected to continue for several weeks.