Police were called to reports of a woman armed with a kitchen knife in a park near Collins Street in the inner city suburb of Thornbury about 2.50pm today.
A witness told 9News a police officer fired their gun after the woman allegedly ran at them.
They used pepper spray before firing a shot, police said.
She was arrested and taken to hospital with serious injuries to her thigh.
The Professional Standards Command will oversee a police investigation into the incident, Victoria Police said.
