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Witnesses in the trial of Chris Kahui accused of the murder of his twin baby sons have described two separate incidents where the twins' mother was allegedly violent with two of her children.

The two witnesses -- one a whanau support person at Middlemore Hospital and the second a cousin of Kahui -- gave evidence today at the High Court in Auckland, about the two separate alleged incidents.

Both witnesses were challenged by crown prosecutors over whether the incidents actually happened.

Kahui is charged with the murder of his twin sons Chris and Cru Kahui in June 2006.

Defence lawyers say someone else, probably the twins' mother Macsyna King, caused their fatal injuries.

This morning Manaaki Poto broke down as she described watching Ms King "dump" one of the twins in his cot, while they were in the neonatal unit at Middlemore Hospital after they were born prematurely in 2006.

Mrs Poto described a one-sided argument between Ms King and Kahui which began with Ms King "screaming and swearing" at him.

Ms King had one of the twins and "dumped" him in the cot causing Ms Poto to stand in between the couple.

Mrs Poto said she looked at Kahui to make sure he wasn't going to retaliate before turning back to see Ms King grabbing the baby and shoving a nappy roughly underneath him.

Mrs Poto cried as she showed the court how Ms King had dumped the baby in the cot and had roughly handled the child.

Under-cross examination crown prosecutor Richard Marchant accused Mrs Poto of lying about the incident, saying if it was as serious as she was alleging she would have told her general manager or staff nurse.

Instead, Mr Marchant said, this was the first time Ms Poto had mentioned witnessing Ms King dumping one of the boys in the cot and had failed to make any notes.

Ms Poto said while she thought she had made a note of the incident she had not told anyone because she had been confident she could work with the family.

Mr Marchant challenged her over her version of events, saying the incident did not happen.

"It did happen I know what I saw," Mrs Poto replied.

Mr Marchant repeatedly asked Mrs Poto if the incident really happened or if the incident was as serious as she was now making out.

Mrs Poto said she did not know why she did not tell the police but that she had to live with her decision, accepting if she had alerted authorities the outcome may have been much different for the twins.

Mr Marchant said Mrs Poto was potentially related to Kahui, through her nephew marrying a woman with the surname of Hetaraka, Kahui's mother's surname, and she was protecting family.

Mrs Poto denied any knowledge of a family relationship, saying not all Smiths were related.

Under re-examination defence counsel Lorraine Smith asked what Mrs Poto's reaction to being called a liar was.

"Of course it happened, I wouldn't sit here otherwise," Mrs Poto said.

Eugene Hetaraka, Kahui's cousin, described visiting his cousin Tracey in Auckland in 2005 and witnessing an incident where Ms King arrived at the house.

Mr Hetaraka said Kahui was at the house with his son Shane, then aged about three months old when Ms King "stormed in" and was "reeking" of booze.

Ms King grabbed Shane "yanking" the boy out of Kahui's arms.

Shane was "screaming his head off" because Ms King had almost pulled his arms out of their sockets, he said.

After Ms King looked like she was taking a swing at Kahui, she stormed out of the house with Shane, knocking his head as they went through the door, he said.

Shane was gasping for air and Mr Hetaraka said he watched as Ms King "threw him in the car", without buckling him into a carseat, he said.

Under cross-examination Mr Hetaraka denied exaggerating the incident in order to help his cousin.

The trial continues tomorrow.