[2][3] Aust joined the Federal Army of Rhodesia and Nyasaland in 1961 as a part of his National Service where he was selected for officer training.

[2] He was appointed as a Member of the Legion of Merit in the Military division for his role in assisting the RLI to adopt fireforce tactics.

On his first day, he published a memo for all RLI soldiers in casual terms introducing his expectations and describing himself as having a "Seedy moustache, glasses and intensely ugly" so they knew how to identify him as their commanding officer.

[8] After the dissolution of the RLI, Aust briefly moved to South Africa, but returned a year later to take over the family farm in Essexvale.

However he eventually had to leave Zimbabwe in 2001 due to his farm being forcibly taken during the Robert Mugabe-authorised violent land seizure.