Police said a man entered the service station with a large knife around 11.45pm Saturday.

A second man entered the service station at 5.45am yesterday, also armed with a knife that was described as a 14-inch wooden-handled ``Bowie'' knife.

In both incidents police said the men demanded cash and cigarettes, which they were given before leaving the service station on foot.

Police described the first man, estimated in his early 20s, as being 165-170cm tall, of a slim build, with a fair complexion and brown hair.

He was wearing a blue baseball cap with white writing on the left hand side, denim jeans and a black top with red sleeves, which had white writing on the back.

The second man, also estimated in his early 20s, was said to be 170-175cm tall, of a slim build, wearing a red hooded windcheater with a faded white logo or writing on the front, baggy jeans, and with brown hair in a mullet-type style at the sides and back.

Anyone with information on either incident is asked to call Glenorchy CIB on 6230 2881 or Crime Stoppers on 1800 333 000.