He showed that Souths fullback Greg Inglis still has plenty of work to do to take his mantle as the game's most complete No.1, with the giant Inglis kept comparatively quiet.
The first try was scored in the seventh minute by veteran Ryan Hoffman in a bizarre piece of play.
Hoffman appeared to be momentarily knocked out when his head hit a teammate's hip while making a tackle and the second-rower stumbled around in a daze.
But an Inglis pass went to ground and was scooped up by Slater who dashed 50m and in the next set Hoffman, who had shrugged off any assistance from the trainers, opened the scoring for the home side.
Souths looked shell-shocked in the first half as Melbourne made easy metres out of their own half, with their attack firing on all cylinders again.
Mahe Fonua became the first Victorian player to play in an NRL final and got his start when Anthony Quinn was ruled out with a calf injury.
The 19-year-old injected enthusiasm and speed in a memorable night as he also got on the scoreboard in the 35th minute to extend the lead out to 18 points.
Any hope the Rabbitohs had of a second half revival was quickly snuffed out when Melbourne repeated the set play from a scrum that had put Fonua over; this time on the other wing with Sisa Waqa over for his seventh try of the season.
The visitors kept Melbourne from crossing again and finally broke their duck through second-rower Eddy Pettybourne in the 72nd minute.
The only bad news for the Storm was forward Kevin Proctor put on report for a lifting tackle on Sam Burgess late in the game while prop Richie Fa'aosa was taken from the field on a stretcher after a head-clash.
Souths centre Matt King was also expected to require a x-ray on a forearm injury.
-AAP