A woman was allegedly undressed and sexually assaulted after falling asleep at a train station in Sydney's inner west this morning.

Police said the 18-year-old was asleep on a bench at Newtown train station when a man allegedly took off her clothes and began sexually assaulting her about 5.10am.

The cordoned-off bench at Newtown station this morning.

A commuter called police, who arrived a short time later and arrested a 28-year-old man from Quakers Hill.

He has been charged with sexual assault and possessing a prohibited drug and has been refused bail to appear before Newtown Local Court tomorrow.