A 29-year-old mother faces a murder charge for leaving her baby in a bath.

The woman, who cannot be named, initially faced a manslaughter charge in relation to the death of her 13-month-old son.

Yesterday, that charge was upgraded to murder when the woman appeared in the High Court at Auckland.

Police allege she left the infant alone in a deep bath for 15 minutes.

The baby and his 5-year-old sister had been living away from their parents for eight months, and returned to them only two days before his death.

Detective Sergeant Megan Goldie, of the Waitakere child abuse team, said the charge was reconsidered when Crown prosecutors reviewed the case.

She said the prosecution would hinge on the woman's intent at the time she left the baby in the bath.

Court documents record that the woman told police she experienced a "rollercoaster of feelings and emotions" when she learned that the children were to return home to her care.

She was unsure she was going to be able to cope, had been unable to sleep and was not feeling emotionally strong.

The children returned to the New Lynn house on November 6.

Two days later, after the woman's husband left for work about 8am, she ran a deep bath for her son.

According to the documents, she told police the water level of the bath was above his nipples and below his shoulders.

She said she placed him in the bath and left the room for about 15 minutes.

When she returned, she found the boy face down in the water, limp, blue and lifeless.

She took him out of the bath and shook him, dried him, wrapped him in a towel and shook him again before placing him on her bed and covering him with a blanket.

Police allege the woman had attended first-aid courses and was a trained dietitian who had worked in hospitals in Fiji, giving advice to mothers and young children on childhood nutrition and development.

She and the children came to New Zealand from Fiji early last year to join her husband.

Police say that despite her medical knowledge and training, she made no attempt to resuscitate the infant or call emergency services or her husband.

When police interviewed the woman, she offered no explanation as to why she left the child unattended in the bath despite knowing he was unstable.

She had seen him toppling over in the bath two weeks earlier.

The woman was remanded on bail for trial in August.