Luisa Yaranamua scored and assisted for Fiji, whose kicking game helped put Australia under pressure in Sunday's semi-final.
Australia recovered from an 8-0 deficit to win 30-14, overcoming its largest shortfall in a decade and qualifying for a fifth consecutive Women's Rugby League World Cup final.
Abbi Church scored four tries, while Tamika Upton scored once and set up two tries, as did Tarryn Aiken.
Jaime Chapman also scored, taking her tournament-leading tally to 10.
Fiji reached its first semi-final after wins over Papua New Guinea and France, with its only group-stage loss coming against New Zealand.
Yaranamua scored Fiji's first try after Sereana Naitokatoka stripped the ball, following Yaranamua's penalty goal.
Australia levelled before halftime, then Church put the Jillaroos ahead in the 39th minute and Chapman extended the lead four minutes later.
Australia conceded two tries in a match for the first time since 2018, after giving up only one try across three group-stage wins.
"We knew (Fiji) start hard and fast and they play this really exciting football that really challenged us," Australian coach Jess Skinner said.
"It's more pleasing to see what the girls are made of in those moments. As a coach, I'm really proud of them".
Fijian coach Jone Wesele said, "the girls really made them run for their money," and added, "They did make a statement today and they've showed the world of rugby league that they're not just here to make numbers, they are here to make the name of themselves."
Australia will face arch-rivals New Zealand in next Sunday's final at Suncorp Stadium.