Dunedin's Cadbury chocolate factory has been evacuated this afternoon.

It's believed a hazardous gas leak has caused the evacuation.

There is a light smell of ammonia drifting around on the Castle Street side, which has six emergency vehicles and a large cordon in place.

A cordon is also in place around the Cumberland street side which is the evacuation site for staff and visitors to the building.

A fire communications spokesperson said they were called at 1.15 and discovered the chemical ammonia.

The spokesman said crews are currently working to track down and isolate the leak.