``A man entered the dairy and threatened a staff member, before running off empty-handed when he realised there was someone else in the store,'' a police spokeswoman said.
The offender is described as a short, slim Caucasian male in his early 20s, who was wearing a black raincoat, black sunglasses, black pants and carrying a big black sports bag.
He is believed to have had a knife bigger than a kitchen knife.
It is understood that shop staff locked themselves inside the premises at the request of police, immediately after the incident which occurred about 6.45pm.
Police quickly converged on the shop and cordoned it off. Uniformed officers could be seen making inquiries inside the premises.
They studied the shop's security camera footage, which appeared to show a dark-clad figure in the counter area.
A police dog and his handler were later seen working in nearby streets in an apparently unsuccessful effort to track the offender.
Anyone who recognises the man's description, or who has information about the incident is asked to contact Crimestoppers, anonymously, on 0800-555-111.
This was the second similar offence at the dairy in the past three months.
In the earlier incident, police sought a masked assailant who attempted to rob the dairy, after obscuring his face with a bandanna and hooded jersey and demanding cash about 8.45pm on Friday, May 6.
The assailant left the store empty-handed.
Police could not say last night whether there were links between the incidents.
