The Roos had not won a game in almost two months, but easily accounted for the hosts (9-203) by six wickets in the one-day match at Richardson Park.
WA rookie Hilton Cartwright top scored for South Perth with 48 as veteran Wanneroo paceman Gary Atkinson took 5-49.
Roos batsman Rob Richards stole the show with an unbeaten 112 off 38 overs.
South Perth’s Jacob Mulder snared 2-37.
Wanneroo ended their 2014-15 campaign in 11th position, a fall from grace after winning last year’s title.