A Canberra postmaster has described the moment a gunman dressed in industrial coveralls pointed the weapon towards his head, punched him in the face and fired a bullet into the air in the latest of a number of armed robberies.

Security footage shows Manni Singh discussing the day's newspapers with a customer at the Rivett Post Office in Woden on Saturday, when the offender burst through the front door before calmly walking around to the back of the counter.

"I really thought it was a prank. I thought 'are you kidding me?' and he said 'nah tell me where your till is," Mr Singh recounted.

Mr Singh said the man "held the gun to [his] head" before demanding cash.

"I said 'you're joking right?' and then suddenly he moved his gun from his right hand to his left hand and he punched me in the head," he said.

Concerned for the welfare of the customer in his store, Mr Singh opened the cash register and the gunman stuffed about $1,500 into a green plastic bag he was carrying.

"On the way out he said 'thank you'," Mr Singh said.

Mr Singh had the gun pointed at his head during the armed robbery. ( ABC News: Jesse Dorsett )

Mr Singh and a few other witnesses followed the offender out of the shop and watched as he crossed the road and ran up a grass path.

"He fired a shot into the air," Mr Singh said.

The ABC understands a car may have been waiting for the offender, suggesting at least one other person was involved in the crime.

Police said they were searching for a young Caucasian man with brown eyes, a slim build and between 165 and 175 centimetres in height.

"An aggravated robbery is always a concern and something the police take very seriously. Particularly when a firearm is used the potential for serious injury or worse is escalated," Detective Leading Constable Daniel Shaw said.

Sharp increase in armed robberies in ACT

Armed robberies were up 27.4 per cent last financial year compared to the previous 12 months. ( Supplied )

Police statistics released this week revealed there were 121 armed robberies in the ACT last financial year — an increase of 27.4 per cent compared to the previous 12 months.

The number of unarmed robberies almost doubled from 57 to 112.

ACT Policing has recently set up a special taskforce to investigate the crimes.

Mr Singh's shop was burgled earlier this year and the latest incident has convinced him to upgrade the security of the small shop.

"I'm going to move the counter to the very far corner of the shop [so I can see people coming]," Mr Singh said.

"Secondly I'm thinking of getting a glass panel around the counter so people can only pass their letters and bills through the little slide thing."

After recently moving from New Zealand, Mr Singh said he no longer felt safe in Canberra.