Samoa bow out after a disastrous campaign, having failed to win any of their pool games and showing little resistance to Australia's full force at a humid TIO Stadium.
The Kangaroos' highlights reel will be led by Holmes' 90-metre effort for his first try, as well as him finishing a team 60-metre movement for his fourth.
The right winger etched his name in the record books with a dazzling run on the opposite side of the field in the 76th minute.
The avalanche began in the ninth minute, when Holmes was the beneficiary of a dubious Josh Dugan tapback from a Cooper Cronk kick.
Holmes' second was far more impressive, breaking out of a tackle on his 10-metre line and offloading for Cronk before getting the ball back and racing away for a long-range try.
Slater rejoined Jarryd Hayne as leading tryscorer in World Cup history off a set play. Morgan clinched his two tries in the space of five minutes.
First, the Kangaroos' five-eighth sliced through some soft defence in the 31st minute, before he won the race to a Cameron Smith grubber with no Samoan defender in sight.
-AAP