A Wellington mother says she thought she would be convicted of killing her son right up until the not guilty verdict was read out.

Speaking outside the High Court in Wellington today, she said she wanted to warn other parents after her 13-month-old son drowned while she was speaking on the phone for eight minutes.

"I didn't know, I should have, but I didn't," she said.

While she felt she was not morally responsible for her son's death she was sure she would fall on the wrong side of the law and be charged with manslaughter.

"My faith in the justice system is restored."

After the verdict, she hugged her family who were in the public gallery throughout the trial, including her young daughter.

"It is so good that it is over and we can to focus on his life instead of his death."

Her mother said the nearly a year since the boy's death in November had been hellish for the family but the stress of the trial was nothing compared to losing her grandson.

The mother has permanent name suppression.

In the High Court in Wellington this afternoon, a jury of seven men and five women found the woman's fatal mistake -talking on the phone for eight minutes while her child bathed - not bad enough to warrant a conviction for manslaughter.

In her closing argument in the High Court in Wellington this morning, Crown prosecutor Sally Carter that said by leaving her child in the bath, the woman made a major departure from the reasonable standard of care.

At several points she had the opportunity to take her child out of the bath, but chose to take the risk, she said.

"The risk is so huge that at the very least a young child of that age should be in constant line of sight," Carter said.

But the woman's lawyer, Noel Sainsbury, said it was wrong to call a tragic mistake caused by genuine distraction a criminal act.

"A terrible mistake of this type is being made every day in this country, but this time it had tragic consequences. In the vast majority of cases it doesn't," he said.

During the trial, the jury had heard evidence that in November last year the mother got out of the bath, leaving her son in 18 centimetres of water, and went to the nearby bedroom to get dressed.

She checked on her son, who appeared fine, and returned to bedroom to strip the sheets. The woman checked on her son once more and walked into the kitchen to call her father.

He was in a rush and told her to call her mother back, which she did soon after. By the time she had finished speaking to her mother, she had been away from her son for eight minutes.

In an interview with police played in court, the woman said she heard her son during the first but not the second call, which lasted more than six minutes.

"It makes me sick to my stomach to think he could have been lying there for six minutes."

After the call, she realised she had taken too long and rushed to the bathroom to find her son limp, cold and blue, floating in the bath.

He was taken to Wellington Hospital before being transferred to the intensive care unit of Auckland's Starship children's hospital.

His life support was turned off four days later with his brain damage deemed too severe.

In the interview with police the woman said she believed her son would be safe for a minute in the bath.

"I thought I would be one minute and I would get off but then I got distracted. I got distracted and he died."