The tourism board has previously used different agencies in different territories, but has consolidated its business into Carat. MediaCom is the incumbent in the UK.
The initial long-list of 10 agencies was narrowed down to three, with the Aegis-owned Carat emerging triumphant.
Carat International chief executive Susannah Outfin said: "It's been a long pitch, as it has been truly global with teams Down Under and Hong Kong and all around the world."
Outfin said that Aegis' digital wing Isobar and research division Synovate had been key to winning the business.
The Carat boss said that winning government business like Tourism Australia has a lot of kudos. "There is a lot of prestige."
And after Carat picked up the UK account for COI TV earlier this year, Outfin added: "It's another big government win."
Carat Australia CEO Richard Halmarick led the pitch and Outfin said the win was "huge for our Australia team".