No8 Seta Koroitamana got a double for Mid Canterbury, but scoring only one other try left them at long odds for the trophy game, while Buller first-five Andrew Norton-Taylor landed a drop goal in the first half.
Putting up a big score over winless visitors wasn't a problem for North Otago, who would ultimately book fourth place courtesy of a 36-5 win in Oamaru to end King Country's hard season.
After a competitive first half, the hosts scored their fourth try in the 60th minute for 22-5, and then blew out the scoreline with 14 more points inside the last nine minutes.
Second-five Matatini Feke got a double, as his team moved a point ahead of Mid Canterbury.
But there was still a game underway in Gisborne, where Horowhenua-Kapiti and Poverty Bay played out perhaps the season's most thrilling match.
Two points above Mid Canterbury and North Otago going into the last round, Horowhenua-Kapiti just needed the win to be safe for the Lochore game, and with three minutes left it appeared they were in, leading 33-29.
In very wet conditions, Poverty Bay had out-scored the visitors four tries to three, but the unerring boot of Horowhenua-Kapiti halfback Jack Tatu-Robinson, with 18 points, had the final in sight, helped by prop David McErlean getting a double.
But Poverty Bay got a succession of penalties to camp right on the tryline, and nearly two minutes into injury time halfback Mario Counsell dived through on the ruck blindside for the match-winner, 34-33.
The devastating loss on a tight points table meant Horowhenua-Kapiti fell to seventh and missed out on any game this weekend, while Poverty Bay will host the playoff for fifth against Mid Canterbury, having the better points differential after not playing the Cantabrians previously this season.
That game will be for the new Bill Osborne Shield, which going forward will become like the Ranfurly Shield for Heartland rugby.
Thames Valley's defeat meant South Canterbury were in no danger of losing their Meads Cup final hosting rights, but the Timaru union still made sure of an unbeaten round-robin with a 46-19 shutout of West Coast in Greymouth.
Far different to the 23-19 thriller two years ago in the Lochore Cup final, this time South Canterbury pulled away after leading 22-14 at halftime, scoring four more tries.
Wingers Kalavini Leatigaga and Sireli Buliruarua both got doubles.
South Canterbury are aiming to win their first Meads Cup and have only the third undefeated season in Heartland history, joining the 2008 and 2016 Whanganui sides.
In the game which had no bearing on the play-offs, East Coast closed out their best campaign since 2012 with a 34-19 win over Wairarapa Bush in Masterton.
With coach Hosea Gear not playing, he looked to his young brother and 19-test veteran Rico Gear to join the bench – making him the third former All Black and fourth Super Rugby player to don the sky blue jersey this season.
The visitors scored five tries, with centre Tawhao Stewart getting a double.
Round 8 results, November 6
Mid Canterbury 22 (Seta Koroitamana 2, Michael Hennings tries; Tom Reekie pen, 2 con) bt Buller 3 (Andrew Norton-Taylor drop goal). HT: 8-3.
South Canterbury 46 (Kalavini Leatigaga 2, Sireli Buliruarua 2, Vaka Taelega, Cam Russell, Henry Bryce; William Wright pen, 4 con) bt West Coast 19 (Amena Tukana, Jacob Lowe, Elliot Smith tries; Sean McClure 2 con). HT: 22-14.
North Otago 36 (Matatini Feke 2, Hayden Todd, Tony Taufa, Levi Emery, Ben Paton; Able Malalogo 2 con, Owen Davey con) bt King Country 5 (Leveson Gower try). HT: 5-0.
Poverty Bay 34 (Ted Walters, Keanu Taumata, Paroraian Manuel-Harman, Andrew Tautavalau, Mario Counsell tries; Manuel-Harman pen, 3 con) bt Horowhenua Kapiti 33 (David McErlean 2, Tautau Kapea tries; Jack Tatu-Robinson 4 pen, 3 con). HT: 17-8 Horowhenua Kapiti.
East Coast 34 (Tawhao Stewart 2, Jorian Tangaere, Hone Haerewa, Ngarohi McGarvey-Black, Sam Parkes tries; Te Rangi Fraser 2 con) bt Wairarapa Bush 19 (Tupou Lea'amanu, Tafa Tafa, Matt Masoe tries; Tipene Haira 2 con) HT: 19-7.