A woman has been impaled by a fence post outside an Eden Terrace bar.

She was impaled by the leg near Galbraith's Alehouse at the intersection of Nikau St and Mt Eden Rd about 10 o'clock last night. 

Fire crews "used tools" to free her, Northern fire service communications shift manager Jaron Philips said. 

It was unclear how the woman was impaled. 

She was taken to in Auckland Hospital in a moderate condition, a St John ambulance spokeswoman said.​