A woman has been charged with murder after the body of a two-year-old girl was discovered inside a house in Sydney's south-west, police say.

The body of the toddler was found inside the bathroom of the weatherboard home on Merino Street in Miller just after midday on Tuesday, after a family friend became concerned after she had not seen the child or her mother for several days.

A girl, 2, was found dead in home in Miller. Credit:Nine News Sydney

The friend initially asked a neighbour for a ladder to scale a fence, before she was able to get into the house and discovered the child's body.

Detectives launched an investigation into the girl's death and, on Tuesday evening, went to a hospital in the Blue Mountains where they arrested a 36-year-old woman.