A woman has been shot by police in Sydney's west.

A woman believed to be aged in her 20s has been shot dead at a fast food outlet in West Hoxton in Sydney’s south-west, police say.

Police declared a critical incident after the shooting at the Hungry Jack’s outlet on the corner of Hoxton Park Road just before midday.

A critical incident means police were involved in the death in some way.

Police said the woman had a knife.

Witnesses have told the ABC they saw emergency services performing CPR on a female.

Police said the woman was assessed by ambulance paramedics but died at the scene.

A critical incident team from the State Crime Command’s Homicide Squad and investigators from South West Metropolitan Region will now investigate all circumstances surrounding the incident, including the woman’s actions prior to police intervention and the discharge of a firearm.

That investigation will be overseen by the Professional Standards Command.

Nicole Maher, whose daughter used to work at the Hungry Jack’s outlet, said she was worried she may know the person who was killed.

“It just scared me,” said Ms Maher.

“I had to come down and make sure it wasn’t my family or someone I knew, and now I’m here I’m just terrified of the fact that a young girl’s been killed today.”

Road closures and traffic diversions remain in place.

This article originally appeared on the ABC and was republished here with full permission.