A mother who allowed her daughter to drown in the bath will be released from prison in less than two months after a judge sentenced her to a minimum of three years and two months in jail.

The woman, now 27, was convicted of manslaughter for the death of the 17-month-old girl in a Wollongong suburb on July 21, 2009, after a NSW Supreme Court jury found her not guilty of murder. Neither can be named for legal reasons.

The toddler drowned after her mother put her in the bath and left the water running while she closed the bathroom door and sat in the lounge room.

After finding her daughter floating in the tub, she dressed the toddler in a christening gown and lay her on a bed.

It was two days before she told anyone of the child's death. She had slept beside the dead child and attempted suicide several times.