An armed robber was found guilty of conspiring to rob a south-side supermarket after his plan was thwarted by a police phone tap.

The 34-year-old criminal and his partner-in-crime were caught scoping out the Kambah Village Woolworths in January last year.

They were taken into custody after members of the Tactical Response Group ''jumped out of some bushes''.

The man's co-offender pleaded guilty and received a lighter sentence in exchange for giving evidence for the Crown at the other man's trial.

The witness also received indemnity from prosecution for a string of unconnected crimes committed throughout Canberra over several years.

Neither the man on trial nor the witness can be named for legal reasons, so are called persons A and B respectively.

Person A has an extensive criminal history, including convictions for armed robbery.

Police swooped on the pair after intercepting calls in which Person A talked about having a look at ''a little something'' the previous night and watching the store staff count the takings through binoculars from nearby.

The pair was also recorded discussing wet weather emergency calls the night before, to which Person A said: ''But there'll be one emergency call tonight, don't you worry about that, that's how I f---ing roll.''

The ACT Supreme Court jury took about four hours on Wednesday to find Person A guilty of conspiring with Person B to commit aggravated robbery.

The Crown's case was that the pair armed themselves with a knife and a pinch bar that they intended to use during the raid.

In his interview with police after his arrest, Person A maintained he brought the pinch bar to break a window and never intended to use it as a weapon.

He told investigators he intended to carry out a ''smash and grab'', and that he knew the law about armed robbery.

Person A claimed to be unaware Person B had a knife and was ''shattered'' to learn he had the weapon. He also said he didn't really know him - an acquaintance of three years - and ''he could have been a f---ing serial killer for all I know''.

But Person B said Person A knew about the knife, and that two balaclava-wearing men with a pinch bar could inspire ''shock and awe''.

Person A remains in custody, and Justice John Burns adjourned the case to February for a sentencing hearing.