Man arrested over Sydney sex attack

A young man has been arrested and is expected to be charged with the sexual assault and robbery of a 46-year-old woman as she walked to work in Sydney's west.

The woman was near an overpass stairwell in Rooty Hill on Thursday evening when the man grabbed her, threatened her and then sexually assaulted her.

At the time, the attacker was described as a teenager, but on Wednesday police said a 23-year-old Doonside man handed himself in.

He is expected to be charged with three counts of aggravated sexual assault and aggravated robbery and is due to appear before Mt Druitt Local Court on Wednesday.