A two-year-old was forced to smoke marijuana through an asthma inhaler by his step-father, with his mother saying she was unable to stop the abuse.

JJ's mother Josephine Lawrence, 27, told the High Court at Auckland her toddler JJ Lawrence was left sleepy and with blood-shot eyes after the sessions with his step-father Joel Loffley, who is accused of murdering the boy by hitting him in the stomach.

Lawrence said Loffley, a marijuana dealer, ignored her protestations and "said it was alright cos he used to get his other sons stoned".

She said Loffley would hold spotting knives under the inhaler for her son to smoke, though it only happened when she was absent.

The shocking evidence sent ripples through the court room and several family members left visibly upset.

A man approached the glass partitioning Loffley from the public gallery and said to him "Big man, eh?"

Lawrence, testifying from behind a screen, said Loffley claimed to want to "bond" with her son but he admitted to possibly breaking the boy's arm while they were "playing rough".

Loffley is accused of murdering JJ on November 14, 2011, by hitting the boy in the stomach so hard it split his liver and pancreas in two.

On the morning of JJ's death, Loffley was play wrestling with the boy on their bed in their Orakei home, Lawrence said.

She was going out and wanted to take JJ with her but Loffley insisted the boy stay with him as he wanted to "bond" with him.

When she returned Loffley had JJ in the shower with the boy crouched over at his feet, a large graze on his chin, allegedly from a fall from the bed.

Loffley put the boy to bed and went out and Lawrence went to her room and watched a movie.

Loffley later came into her room carrying a limp and unconscious JJ.

"He said 'there's something wrong with this boy, we have to take him to the hospital'.

"I was freaking out, touching him trying to wake him up. I was saying to him 'What the f... have you done? I hate you'.

"I went into shock mode, I just started nutting out."

The Crown case is that Loffley assaulted JJ while his mother was out.

Loffley told Lawrence JJ had fallen off a bed he had been making and JJ had hit a wall and his legs had gone over his back.

A post-mortem found JJ had been hit so hard in the stomach his liver and pancreas had been forced back onto his spine and had split in two.

He also had cannabis in his system.

Lawrence said Loffley was violent with her on numerous occasions including punches, kicks and strangulation.

He strangled her severely once when she disobeyed his cooking instructions, and he would confiscate her money and force her to do all the housework, she said.

JJ suffered a second broken arm a few months before his death.

Lawrence said she did not take her son to the hospital when it happened because she was drunk and she did not want JJ taken from her.

She called an ambulance the next morning and said JJ had fallen from a jungle gym.

They took the boy's cast off early, she said.

"Joel said it would be alright - his other sons had had broken arms."

Defence lawyer Roger Chambers said it was "not [Loffley] in that household" who had hurt the boy.

In each cross-examination, Chambers has focused on the behaviour of Lawrence.

He is due to cross-examine Lawrence today.

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- © Fairfax NZ News