"I was just looking at this barrell and I was thinking 'Please don't pull the trigger'," he said.

Dylan Elias Keenan, 32, went on trial in the ACT Supreme Court on Tuesday accused of the aggravated robbery at the Chapman IGA.

He has pleaded not guilty, and is arguing he was not the balaclava-wearing man responsible for the crime.

The Crown alleges Keenan, of Cooma, stormed the supermarket armed with a loaded gun on August 19 last year.

CCTV footage, played to jurors, shows a staff member trying to close an external door.

Seconds later a masked gunman bursts in and strikes him in the face, causing him to fall to the floor.

The robber then used the shortened firearm to demand cash from another store employee, before fleeing with $1500.

Both employees gave evidence on Tuesday, and the man who handed over the cash spoke of his fear during the ordeal.

"I was just thinking of my little girl at the time. She's only three and I thought 'I don't need this'," he said.

He said he had begun to worry when he saw the gun but couldn't see his colleague.

"I saw [the colleague] lying on the ground and I thought 'Oh my God'," he said.

The armed offender forced him to hand over money from tills and it was put into a green shopping bag.

The court also heard evidence from a jogger, who said she found a black balaclava near the scene and reported it to police.

The trial will resume before Justice John Burns on Wednesday morning.