They could thank first five-eighths Willie Wright's goal-kicking class as they scored only two tries to three scored by the home team.
South Canterbury were down 17-19 with nearly 27 minutes remaining but penalty goals landed by Wright in the 55th and 68th minutes saw them home.
Down 5-17 at halftime, West Coast scored two converted tries in nine minutes to claim a 19-17 lead.
It is the first time since 2013 that South Canterbury have held the Cup – West Coast have never won it.
South Canterbury captain Miles Medlicott told SKY Sports, "It was pretty tough in the conditions but it was the same for both sides but we did enough to get away with the win.
"Playing into the wind in the second half suited the way we wanted to play. We wanted to keep it close and rumble it up so it worked in our favour in the end.:
West Coast captain Troy Tauwhare said: "Conditions were pretty tough for both teams today and South Canterbury started really strong. We knew it was going to be a tough battle. We came back well in the second half but South Canterbury showed what a class outfit they were and got the win in the end.
Scorers: West Coast 19 (Amentatave Takana, Troy Tauwhare, Daniel Foord tries; Jesse Pitman-Joass 2 con) South Canterbury 23 (Anthony Amato, Kalavini Leatigaga tries; Willie Wright 2 con, 3 pen ). HT: 5-17