The Kangaroos scored eight tries to three to book a place in the Wellington decider against a Kiwi side yet to drop a game in the tournament.
Man-of-the-match Greg Inglis scored two tries as did Cooper Cronk, while Daly Cherry-Evans, Dave Klemmer, Josh Mansour and Josh Papalii also crossed for the hosts.
David Fa'alogo, Ben Roberts and Tim Simona replied for Samoa.
Australia needed to win the game to reach the final while the Samoans were chasing a victory by nine points or more.
The Kangaroos led 28-6 at half-time and withstood a brief second-half fightback from Samoa to secure their 26-point win in front of 18,457 fans.
New Zealand grabbed top spot in the tournament standings after beating England 16-14 in Dunedin on Saturday.
The Kiwis beat Australian 30-12 in the tournament opener but will go into Saturday's final as slight underdogs.
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