A Marton mother whose toddler drowned in a swimming pool in her absence has been discharged without conviction - believed to be the first time granted in a manslaughter case in New Zealand.

In January this year, Mary-Anne Illston's 22-month-old daughter Hannah Thomsen drowned in a pool while she went inside the house briefly to attend to her newborn baby.

She pleaded guilty to manslaughter in the High Court at Wanganui last month.

Illston, a nurse, left Hannah and other children at the pool without supervision after she heard her two-week-old baby crying inside the house, 15m away, Justice Forrest Miller said at sentencing in the High Court at Wellington today.

Advertisement

Illston had intended to bring the newborn back out to the pool but stayed to change the baby.

A very short time later she returned to find Hannah had drowned in the deep end of the pool.

Justice Miller said Illston had left the pool only briefly, with the intention of bringing her baby back at once.

She was also "exhausted and overwhelmed", having just had a newborn and had recently lost her mother to cancer.

Justice Miller said water was "notoriously dangerous" and children needed supervision.

"Your lack of care has had a terrible consequence, as you know better than anyone. You also knew that Hannah could not swim and had fallen into the pool the previous day."

In granting a discharge without conviction, he noted that his decision was based primarily on the impact a conviction would have on Illston's family.

He said he was aware of no other case in which a discharge without conviction was granted for manslaughter in New Zealand.

Advertisement

Manslaughter covered "a vast range of wrongdoing" and Illston's departure from the expected standard of care was at the lower end of the scale.