The failed robber jumped the counter of the Invermay fast food restaurant in the state's north on Monday night and began grabbing money from cash registers.

But he got a little more than he bargained for.

"Whilst he was doing that a member of the public has struck him with a chair," Sergeant John Parker told reporters.

The slim man, who was wearing all black clothes and orange material across his face, then ran out of the store empty handed.

He was brandishing a kitchen knife about 20cm long, police said.

Staff at the store were left shaken but no one was hurt, while the Good Samaritan has remained anonymous.

"Although we wouldn't recommend people take action, we have no issue with the action he took," Sgt Parker said.

Police believe the attempted robbery may be linked to another at a Pizza Hut at nearby Mowbray earlier in September.

They're asking anyone with information to come forward.

Australian Associated Press