Wanganui will defend their Meads Cup title against Horowhenua Kapiti next weekend while Mid Canterbury will play West Coast for Lochore Cup honours.
Perennial Meads Cup finalists Wanganui will be looking to add another title to the five they have won since 2006.
Trailing 20-10 at half-time they edged top-of-the-table South Canterbury 29-24 in Timaru despite the absence of key backs Craig Clare and Timoci Seruwalu.
In the other semi-final, second-placed Horowhenua Kapiti took the narrowest of wins 18-17 over third-ranked Buller in Levin, James So'oialo landing six penalties to secure the win.
In the second-tier Lochore Cup, West Coast accounted for defending champions North Otago 24-14 in Greymouth, running in four tries to two after leading 17-7 at half-time.
They'll face Mid Canterbury who ran in eight tries in thumping Poverty Bay 56-22 in Ashburton.
Fijian winger Willie McGoon bagged a hat-trick, while Sam Finau and Maleli Sau picked up two apiece.