Prisoner Anthony Charles Mare, 23, had previously pleaded guilty in the Supreme Court in Launceston to aggravated armed robbery.

On Wednesday, Crown prosecutor Luke Brett told the court that Mare was a passenger in a car on the night of July 21, 2014.

His co-offenders in the car were Shaun Lillico, then 20, a 16-year-old youth and a 17-year-old girl.

All three had previously pleaded guilty to their crimes and have been sentenced.

Mr Brett said the car was used in the armed robbery of a Mowbray Nepalese restaurant, the Himalayan Kitchen, but Mare had no involvement with this crime and was not charged over it.

However, the prosecutor said Mare did have involvement with the second armed robbery, this time at the Kings Meadows Blockbuster Video store later on the same night.

Mr Brett said Mare stayed in the car, acted as look-out, allowed the principal offender to borrow his jumper to commit the armed robbery, and opened the car door for the others after the robbery to enable them to make a quick getaway.

The offenders who entered Blockbuster used a tyre iron to threaten the lone female cashier, then 21, robbed her of her new $950 mobile phone and stole about $190 cash from the register.

Defence counsel Fran McCracken, in her plea in mitigation, told the court that her client played a very minimal role in the armed robbery, which would have happened whether her client was there or not.

Ms McCracken said Mare had fallen in with the wrong crowd who introduced him to drugs, first cannabis and then methylamphetamine, also known as ice. 

She said Mare's drug use had been detrimental to his mental health.

Justice David Porter said he was familiar with the younger two of Mare's co-defendants and had dealt with one of them at least twice.

He adjourned sentencing to December 10.