Police are investigating an aggravated robbery at Balhannah this morning.

At about 5.18am, a man armed with a knife entered the Liberty service station on Onkaparinga Valley Road, and allegedly threatened the shop assistant and demanded money.

He fled with an amount of cash, and left the service station towards the Balhannah Oval.

The suspect is described as Caucasian, aged in his early 20’s, at least 180 cm tall, thin build, blonde hair, brown eyes and an Australian accent.

He was wearing a blue hooded jumper, black track pants, work boots, a material ‘ghost’ style mask covering his face and light blue latex gloves.

The attendant was not injured during the incident.