Eddie Jones’s Wallabies, who have now played all their games, will be eliminated in the group phase for the first time if Fiji take a point against Portugal next week in the Pool C finale.
Portugal led 7-3 after Pedro Bettencourt’s early converted try but the centre was then sin-binned for a high tackle and Australia made the most of it by scoring three tries through Richard Arnold, David Porecki and Angus Bell.
Fraser McReight touched down to secure the bonus point shortly after the break, while Marika Koroibete got a fifth with only a few minutes left.
Ben Donaldson added points from the tee as twice world champions Australia, who conceded a late converted try by Rafael Simoes after picking up two yellow cards, climbed to second on 11 points, three behind already-qualified Wales and only one ahead of Fiji.
