Young NSW mum charged over baby assault

A young mother who allegedly assaulted her 18-month-old baby in Sydney's southwest has been charged following a lengthy investigation.

Detectives from the state crime command's child abuse unit began looking into the alleged assault in February 2015 following reports the child had been injured.

The baby's 21-year-old mother was arrested at Bankstown Police Station on Tuesday afternoon and charged with four offences including grievous bodily harm with intent and assault occasioning actual bodily harm.

She was refused bail at Bankstown Local Court on Wednesday and is due to face Downing Centre Local Court on September 14.

Police say the woman's daughter has been placed in the care of the NSW Department of Family and Community Services.