Waipu countered relatively quickly off a scrum through blindside flanker Karatiana Vette-Walsh before a penalty goal had the visitors up 10-5.
Mid Northern centre James Cherrington then broke the game open. He busted through three would-be tacklers on his way to the line to tie the game up before halftime.
Mid Northern started the second stanza as strongly as they ended the first, with inspirational leader Olsen crossing for his second try off a lineout drive.
Waipu threw everything they had at Mid Northern but the home side refused to budge as a finals spot was in their crosshairs.
First-five Jordan Levavasour ended Waipu's resistance with a hot-stepping effort late in the piece, handing Mid Northern the spot at Toll Stadium.
Meanwhile a superb start to the other semifinal helped OBM book their spot in the final.
The home side started with a hiss and a roar, posting 28 unanswered points in the first 25 minutes. They were led by first-five Peter Breen who had a fantastic running game and kicked four from four.
Coach Greg Shipton said it was the best 25 minutes of rugby he had ever seen.
When the defending champs scored their fifth try in the corner the score stretched out to 35-0. Not long after that Hora Hora crossed for the first of their tries.
The big and boisterous Hora Hora fan base were on hand to cheer their side on. The dominant start didn't diminish their support and noise levels.
When Mitchell Reader intercepted a wayward pass to dot down under the posts, the home side has notched up the half-century, which is almost unheard of in a semifinal match.
Jimmy Hardy finished the game off with a simple try in the corner, providing great delight for the home crowd, however all the hard work was done by man-of-the-match Monty Lama. All Hardy had to do was fall over the line to wrap up the victory.
Meanwhile, Kerikeri have given themselves the opportunity to move into the Bayleys Premiership after beating Mid Western 18-0 in a promotion/relegation battle.
Kerikeri continued their impressive season as they held the Premiership side scoreless on their own ground.
The result is a disappointing end to a tough campaign for Mid Western, who are a long way from the side that won back-to-back Premierships in 2011/12.
Kerikeri can enter the Bayleys Premiership for 2018 subject to meeting NRU criteria.
In the other promotion/relegation match Otamatea comfortably accounted for Moerewa/United Kawakawa 33-7.
Results: Premier:
OBM 64 Hora Hora 17
Mid Northern 24 Waipu 10
Premier Reserves:
OBM 36 Mid Northern 5
Kamo 30 Waipu 25
Promotion/Relegation:
Otamatea 33 Moerewa/UK 7
Kerikeri 18 Mid Western 0
