It was a deserved victory, but not without controversy, as Samoa lost hooker Chanel Harris-Tavita in the 47th minute, knocked out cold by a blatant Angus Crichton elbow.
It was a violent act, that merited a sending off on recent interpretations, but the officials settled for a sin bin, while Samoa were left with one playmaker for the rest of the match.
But Australia had won the game in the first half, absorbing periods of Samoan pressure, before scoring three well-executed tries, with man of the match James Tedesco outstanding.
But Toa Samoa did themselves proud. They overcame a 60-6 opening day defeat and the loss of five players across the tournament to become the story of the event.
They were massive outsiders at Old Trafford ($6.50 at the bookmakers), given that the average margin in five previous encounters was more than 35 points, but they acquitted themselves well.
But any chance of a Samoan upset was undone by untimely errors, while Australia were defensively superb.
The Pacific team dominated the first 10 minutes. They were unfortunate – as a Harris-Tavita 40-20 was missed by the officials, with Tedesco putting his foot in touch as he tried to retrieve it.
They enjoyed sustained pressure but that was released with a poor Jarome Luai kick and Australia broke out, before Latrell Mitchell powered through on the left edge.
In the blink of an eye the Kangaroos were in control and Josh Addo-Carr exposed some poor defence through the middle, before sending Tedesco away under the posts in the 17th minute.
It was all going Australia’s way and Samoa had to withstand four consecutive sets, with Mitchell shelling the ball close to the line.
Samoa then had their best spell. Addo-Carr stopped a certain try with a timely knockdown, before Harris-Tavita was held up under the posts.
But the pressure was again extinguished by a Anthony Milford spill and Australia made them pay, barrelling downfield, before Harry Grant put Liam Martin across from close range. Given the occasion, it was soft try and a turning point of the half, as the huge crowd willed the underdogs on.
Samoa had a half chance but a Luai inspired break down the left was well covered by Tedesco. The Pacific team did well to restrict further damage before the break, repelling multiple sets on their line.
The flashpoint came when Harris-Tavita was levelled by a Crichton elbow. He should have been dismissed, but got away with a sin bin.
To their credit, Australia defended brilliantly with 12 men, before Nathan Cleary sent Cameron Murray with a perfectly timed pass, after Cameron Munster had come up with a trademark steal.
Samoa were finally on the board after an hour, as Brian To’o crossed in the corner after a superb Luai cut-out ball.
The 67,000 crowd rose, but a well worked Tedesco try killed any hopes of an unlikely comeback, though Crichton’s 50-metre intercept was reward for a determined Samoan effort.
Mitchell iced their triumph with a slashing 80th minute try, after a bullet pass from Cleary, as Australia celebrated yet another World Cup triumph.
Australia 30 (Latrell Mitchell 2, James Tedesco 2, Liam Martin, Cameron Murray tries; Nathan Cleary 3 cons)
Samoa 10 (Brian To’o, Stephen Crichton tries; Crichton con)
Halftime: 14-0