Police are still looking for the man and want anyone with information to call Crime Stoppers on |1800 333 000.
The robber was handed an amount of cash and left the store.
He is described as being slim, wearing a black hooded jumper, black jeans, white sneakers and had material covering his face.
Franchisee Alvaro Nascimento said security at the store was �always pretty strict�, the store had 10 CCTV cameras and magnetic door locks.
He said this was the first incident of its kind at the current location in the Kwinana Marketplace and staff were all right but there were concerns the man may look for other opportunities.
�Our stores are the only activity at the time of night,� he said.