A woman is fighting for life and a man will face court today after a serious sexual assault in Sydney’s south-west.

Emergency services were called to a park on Lealocks Lane in Casula after receiving reports that a woman was unconscious at about 1.45pm yesterday.

Officers were told a 27-year-old man had allegedly sexually assaulted a woman before leaving the scene.

The 18-year-old woman was treated by paramedics and was taken to Liverpool Hospital in a critical condition.

Following inquiries, the 27-year-old man was arrested on Canberra Avenue in Casula.

He was taken to Liverpool Police Station and later charged with aggravated sexual assault.

The woman is still in a coma today.

The man was refused bail and will face court today.