Samoa's discipline unravelled in the semi-final, with nine errors and 12 penalties or set restarts helping Australia secure a 44-12 win.
Prop Spencer Leniu was sin-binned for dissent late in the first half, as the Kangaroos built a 26-2 lead.
Stephen Crichton opened the scoring with a fourth-minute penalty, but Tom Dearden, Jacob Preston, Tom Trbojevic, Reece Walsh and Bradman Best crossed before halftime.
Walsh, picked to start at fullback ahead of James Tedesco, scored three tries and assisted Jack Bostock's late touchdown.
He celebrated his first try with a backflip, which coach Kevin Walters said was done for sick children.
"That's just Reece, he's always brings up something that you don't expect and the backflip was no different," said Walters.
Walsh briefly erred after halftime, when Crichton gathered a bomb he had failed to claim and scored for Samoa.
Samoan coach Ben Gardiner said Australia benefited from "homeground calls".
"On the back of some of the penalties that were given, we just didn't react very well on the back of it," said Gardiner.
"Then it was one thing into another and then a try, so it made it really difficult to be able to fight back from that."
Walters praised his team's performance and said he believed more improvement was possible.
Australia will meet New Zealand at Brisbane's Suncorp Stadium next Sunday, seeking a fourth consecutive World Cup title after winning every tournament since 2013.