Augustus was granted sole imperium within the city of Rome in addition to being granted proconsular imperium maius and tribunician authority for life. Traditionally, proconsuls (Roman province governors) lost their proconsular "imperium" when they crossed the Pomerium—the sacred boundary of Rome—and entered the city. In these situations, Augustus would have power as part of his tribunician authority, but his constitutional imperium within the Pomerium would be less than that of a serving consul…

“Augustus was granted sole imperium within the city of Rome in addition to being granted proconsular imperium maius and tribunician authority for life. Traditionally, proconsuls (Roman province governors) lost their proconsular "imperium" when they crossed the Pomerium—the sacred boundary of Rome—and entered the city. In these situations, Augustus would have power as part of his tribunician authority, but his constitutional imperium within the Pomerium would be less than that of a serving consul…” is a paper by Seyi published in 2023. It has an Open Access status of “closed”. You can read and download a PDF Full Text of this paper here.