An armed robber who said "please" as he held up the Fyshwick post office with a fake gun in June 2016 has been jailed for more than four years.

Dean Vaughan, 37, pleaded guilty in the ACT Supreme Court to the Canberra hold-up.

He had entered the post office wearing a balaclava and carrying the replica gun.

Vaughan demanded money, emptied three tills and escaped with more than $1,000.

Vaughan earlier said he committed the crime to get money for drugs.

He told the court he tried not to threaten anyone directly with the fake gun, though he accepted staff would have been frightened.

Vaughan told the court since the crime he had re-evaluated his life, mentioning the trauma he experienced while trying to save his cellmate using CPR. The man died in May.

Justice John Burns said Vaughan's rehabilitation commitment must be treated with caution given his significant drug addiction and previous offending.

Vaughan will serve nearly three years non-parole and will not be eligible for release until 2019.