Subway branch owner Colin Prebble said the alleged offender threatened to 'bash' the worker's head in with the rock, during the incident that happened at about 8.40pm.
The offender left with cash.
The store is located on the corner Barbadoes and Cashel Sts.
A 24-year-old man was arrested soon after by police and will appear in the district court on Monday charged with aggravated robbery.
