Goal-kicking proved the difference as the Storm edged Souths 18-12 in a three-tries-apiece affair at the Downer NRL Auckland Nines.
Despite some end-to-end play neither side was able to cross til midway through the first half when Storm forward Kenny Bromwich crashed over out wide. Nelson Asofa-Solomona made it back-to-back for Melbourne moments later after deft passes from Curtis Scott and Cameron Munster.
Winger John Olive finally got Souths on the board despite more impressive defence from the Storm but they were still left facing a 12-4 half-time deficit.
An offload under huge pressure from hooker Damien Cook handed Hymel Hunt an easy run to the line but a second missed conversion left them 12-8 down.
A curious missed field goal attempt from Ben Hampton raised some eyebrows but with a minute and a half left a brilliant 40/20 from Blake Green was followed by a perfect cutout from the same player to hand Curtis Scott the match-sealing try.
"It's great that we are through to the finals because the club has never advanced past the group stage before," Green said after the win.
"It's going well so far, it's been pretty controlled by us and hasn't been very flashy, but we have done enough to get a couple of wins."
Right on full-time Cook raced after a Cameron McInnes kick and toed ahead to score just inside the dead-ball line but it was too late to change the outcome.
South Sydney forward Kyle Turner said it was more than missed conversions that let the Rabbitohs down.
"You could say the goal kicking let us down in that game, but it was pretty much stuff all through the game which we didn't really live up to," Turner said.
We didn't watch their kick-offs and as a result they got a lot of ball and we just couldn't hold them out," Turner said.
"We need to pay attention to the little things and work on our shapes and patterns which we have designed, because we have hardly used them so far."
Melbourne Storm 18 (K Bromwich, Asofa-Solomona, Scott tries) def South Sydney Rabbitohs 12 (Olive, Hunt, Cook tries)