There's been angry denial from twin Karlos Stephens' mother that she was the person who killed her son.

Pamela Stephens also strenuously denied being found with her hands around the neck of another son.

Stephens was subjected to intensive cross-examination this afternoon by Max Simpkins, the lawyer acting for Shane Claude Roberts, 60 who is charged with murdering the infant sometime during the hours of November 29-30, 2014.

He pleaded not guilty when the trial began in the High Court at Rotorua on Monday.

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Simpkins opened his questioning by accusing Stephens of lying to police and the court about a number of matters, including the length of time she spent alone with the twins at Roberts' Alison St house before Karlos died.

He claimed it had been at least 10 hours, she insisted it was only two to three.

"He [Karlos] was grizzly and crying, you couldn't handle the situation ... it was you who assaulted the baby while Mr Roberts was away from the address, that is what happened, isn't it?" Simpkins demanded.

Stephens categorically rejected the allegation he levelled at her.

Simpkins continued his questioning by saying she needed to blame Roberts otherwise the focus for Karlos' death would have been on her.

Again Stephens responded sharply saying "absolutely not".

She denied she'd once asked Roberts to pick the boys up from her Te Ngae flat because she thought they were possessed or that she was having frightening thoughts about harming her babies.

Challenged that someone had to break into a room to get to her finding her with her hands around her five-year-old son's neck, Stephens defended herself, saying she was holding him to prevent him being taken from her.

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Asked when she had last seen her surviving twin, Hosea, Stephens said it had been about 2015 when she bumped into him while Christmas shopping at The Warehouse. He was with Roberts' former wife.

Re-examined by the prosecutor Amanda Gordon, Stephens said she hadn't attended her baby's tangi in Te Whaiti because of miscommunication - she'd thought she was going with her mother who, in turn, thought her daughter was being taken there by someone else.

The jury has begun watching a DVD recording of a conversation a trained child interviewer had with another of Stephens' sons, then five.

He talked of going to Kraut's house with his mother and baby brothers. The court has heard Roberts was frequently known by that name.

Earlier in the day Stephens told the jury Roberts had instructed her to say baby Carlos had died from cot death.

She admitted giving two versions of events surrounding the infant's death to police. who reopened their investigations into it last year.

The trial is proceeding.