The offender eventually managed to flee, but police will allege in court that he is a 23-year-old Evatt man who was subsequently arrested during a raid early on Sunday morning.

Detective Sergeant John Giles said on Sunday that a man armed with a knife "distracted the store assistant" inside the bakery at the Evatt shops before jumping the counter and stealing an undisclosed amount of cash.

He said the staff member chased the robber to the front of the store and asked two Icon Water workers, who were at the shops, for help.

This duo then ran after the offender and caught up with him on a footpath, taking some of the cash from him before he escaped.

The heroic bystanders then returned to the shop with what they had recovered.

"More importantly, [they] were able to gain a very, very good description of the alleged offender," Detective Sergeant Giles said.

After police executed a search warrant in Evatt and apprehended the man they allege to be the offender, officers charged him with one count of aggravated robbery.

The man will spend the night in the ACT Watch House, and is expected to face the ACT Magistrates Court on Monday.

While no one was injured and police were pleased with the "reasonable ending" in this case, Detective Sergeant Giles warned members of the public to be "very wary about taking matters into your own hands and chasing down offenders".

"That doesn't take anything away from the [Icon Water employees], who did a fantastic job," he said.

"We're absolutely grateful for their efforts."

The senior officer also said the shop attendant had been "very shaken up" by what occurred.