A man has been charged with sexually assaulting a six-year-old girl in Sydney's south.

Police say the 51-year-old assaulted the girl in a room during a visit he made to a home at Botany on Sunday.

The Child Abuse Squad launched an investigation on Wednesday after receiving information and arrested the man that afternoon at Five Dock in the city's inner west.

He has been charged with one count each of sexual assault on a person under 10 and aggravated indecent assault.

The man was refused bail and will remain in custody until he faces Central Local Court next week.