Woman discharged for manslaughter of son should not have been prosecuted, lobby group says.

A lobby group is criticising police for prosecuting a Whanganui health professional, whose son died after being left in a car on a hot day, saying it was not in the public interest.

But police say it is their job to present the evidence to the court, which then decides what is appropriate.

The 35-year-old woman was discharged without conviction by Justice Simon France in the High Court in Whanganui on Friday, having pleaded guilty last month to the manslaughter of the 16-month-old toddler, who died on January 16.

The woman, her husband and the child were all granted permanent name suppression.

Family First director Bob McCoskrie said he welcomed the decision, but was disappointed with police's actions.

The case was not in the public interest, and police could have used their discretion to keep the case out of court, he said.

"The drawn out court process has served no public benefit. It has just added to the pain of a mother who has already received a life sentence."

Acting Detective Inspector Dave Kirby of Whanganui Police said the investigation had been "complex and challenging".

"Police are required under law to investigate and consider the circumstances of every death, and we do so objectively, fairly, and thoroughly in the interests of the victims.

"Where there is sufficient evidence of a case to answer, then police presents that evidence to the court for it to make a decision on the appropriate outcome."

While delivering his decision, the said there was "obvious public interest in the matter".

The woman sobbed, and many others in the courtroom - including a court staff member - cried as the judge described how the child died.

Her job was the kind that would "occupy your thoughts, even though you have ostensibly left work for the day", he said.

She had been working long hours for many days, and got home at 12.30am on the day her son died.

She left for work little more than six hours after getting home, putting her son in a car seat so she could drop him off at daycare on the way.

The judge said she was "tired, distracted by work, and somewhat on auto-pilot" when she forgot to turn off toward the daycare centre.

Instead, she parked her car, got out and went into work, her son still sitting in his booster seat in the car.

Hours later she got a text message from the daycare centre, asking if the boy was all right.

An employee of the centre called her, telling her the boy had not been dropped off.

She dropped the phone and ran to her car, but her son could not be revived.

He had died of heatstroke and dehydration.

The judge said there was not an obvious risk to the child, and the woman had an "extraordinary blanking of the mind".

"This aberrant mental state was no doubt in part of a product of her extreme tiredness."

Crown prosecutor Lance Rowe said a discharge without conviction was "probably irresistible".

While there was a collection of strong mitigating factors, and no aggravating ones, the gravity of the offending was high due to the nature of the charge, he said.

"The law places a premium on the care of children and vulnerable persons, and the duties that arise for people charged with their care.

"That is a very serious duty."