A win, a draw, a losing bonus point (scoreline stays within 7 points or fewer), or a four-try bonus point were all the ways Fiji could make it through to the next round.
Portugal won the pool C encounter 24-23, but the losing bonus point is enough to see Fiji through.
It means Australia finishes third in the pool and are out of the World Cup.
Levani Botia and Mesake Doge scored Fiji's two tries.
Fiji will face England in the last eight.
Look back on all the action on RNZ Pacific's blog:
The Flying Fijians will be playing for more than just a win against Portugal on Monday morning [Fiji time] in Toulouse.
Australia did just enough to keep their slender World Cup hopes alive with an unconvincing but hard-fought 34-14 bonus-point win over determined Portugal team in their final Pool C match.
The Flying Fijians survived a scare and hung on to win 17-12 against a spirited Georgia in Bordeaux on Sunday morning, giving them hope of a quarterfinal spot at the Rugby World Cup in France.
Coach Kieran Crowley says Italy will be fully focused for their final World Cup pool game against France after a crushing loss to the All Blacks.
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