Mid Western scored two tries - one from a chip and chase and the other was a charge down - plus a penalty while Mid Northern scored a converted try and kicked a penalty.
Cauty said his team beat Wellsford in round one but got thumped 61-3 about two months ago.
"We worked hard and turned up last weekend and will do the same for the final ... just have to manage a few niggly injuries and work on our discipline."
His side lost to Wellsford in the final in 2009 and Cauty said his players would need to play to their best against a good side.
Loose forwards Pete Mirrielees and Ben Berridge had a strong game, with the former taking up hooking duties in the last 25 minutes.
Wellsford boasts the best defensive record of all Northland teams and co-coach Paul Humfrey reckons it will be crucial in the final. "The only game we lost this season was against Mid Western and it's only fitting that we play them in the final because both teams have been fighting for top position during the competition."
All players were injury free and he expected a big crowd at Toll Stadium.
In other results, the premier reserves final will be between Mid Northern and Western Sharks, after the latter beat Kamo 22-15 and Mid Northern defeated Wellsford 28-8.
In the Bay of Islands final, Kerikeri 32 beat Ohaeawai 15.
