Coast's set piece was much better this week, with centre Luke Hughes given more chances to run the ball than in previous games.
His straight running usually helped him over the gain line, and allowed quick ball for the likes of lock Brad Houston to pick and go quickly, and halfback Dave Woods to swing play in the opposite direction.
Coast opened with two penalties after gaining the upper hand early in the game, but South Canterbury first five-eighth Rick McKenna responded 24 minutes into the first half by slicing through the Coast defence and passing wide to fullback Jarred Trevathen from a scrum set piece.
Houston added to last weekend's brace by forcing his way over from a ruck 5m out, following a patient forward surge by West Coast to reclaim the lead for his side at 13-7 with the conversion.
Second five Sean McClure extended that buffer with two penalties before Thomson was yellow carded, leading to Edwards' try.
South Canterbury threatened late in the match but were not able to break through when Coast had a full compliment of players on the field.
Scorers
West Coast 19 (B Houston try; S McClure 3 pens, con; P Thomson pen), South Canterbury 12 (J Trevathen, L Edwards tries; Trevathen con).
GRS lb