Police are seeking assistance from the public following a spate of chemist robberies over the weekend.

Four chemists were robbed, two on Saturday at Blakeview and Alberton, and two on Sunday, at Northfield and Parabanks.

In all instances, the robber is armed with a knife and is described as a man of Caucasian appearance, aged in his 20s, about 183cm tall, with a slim build and wearing a skull mask.

He has initially fled each scene on foot but may have had a car or bike waiting.

The most recent robbery occurred last night at Parabanks at about 8.20pm when the man threatened staff with a knife and stole cash which he stuffed in his shorts.

He fled on foot and was last seen on John Street heading towards the Coles store.

Witnesses from the Northfield robbery earlier on Sunday afternoon said he was wearing black sunglasses and a bandanna, a zip-up light brown or off-white polar fleece jacket, a yellow polo shirt with an emblem on the right breast, blue shorts above the knee, and white sneakers with black shoelaces.