After being under the pump in the first 10 minutes and conceeding an early try, the Cantabrians crossed for three converted tries to lead 21-7 at the break.
West Coast came back in the second stanza but left themselves short, having to settle for two bonus points.
Powerful No8 Seta Koroitamana got a double for Mid Canterbury, while fullback Dan Fransen also scored for the second week in a row.
Second-five Andrew Letham is the current preferred goal kicker ahead of Jarred Percival – the first-five who sunk Wanganui with the boot last year – and Letham contributed 11 points.
Both of West Coast's import forwards Amenatave Tukana and Tumama Tu'ulua scored, as did local second five Sean McClure and winger Duncan Wood.
Laid low after two losses right on fulltime, Buller had no excuses this weekend as Thames Valley made the long trip to Westport and picked up a big 43-22 victory, leaving the home side already likely out contention for the Meads Cup playoffs.
Buller briefly got the lead 10-7 after 22 minutes, then Thames Valley turned it on to lead 19-10 before the home side scored again through lock Caleb Aperahama to close the gap to 19-15 by halftime.
But two more tries by Thames Valley after the break had them out to 33-15 and Buller did not have enough for a customary fight back, with Thames Valley's important South Island away win helping them lock down fourth spot on the points table.
Reserve back Ethan Seymour came on to score two tries, while Matiu Abraham also got one off the bench as Thames Valley pulled away.
First-five Reece Broughton scored a try and added four conversions.
The Anthony's – Ellis and Tailua – were Buller's other tryscorers.
In a battle that may well decide the wooden spoon, Wairarapa Bush went to Ruatoria and picked up 26-5 win over battlers East Coast, who just cannot break a winless streak that is stretching back to 2013.
Wairarapa Bush prop Sam Gammie scored a double, while first-five Glenn Walters added three conversions.
The final placings for the round were not decided until yesterday as Horowhenua-Kapiti continued their tradition of taking a home game to Wellington's Westpac Stadium, playing a curtain raiser for the Wellington vs North Harbour match.
Screened live on Sky TV, the match proved a fun advertisement for the Heartland Championship as visitors South Canterbury turned it on to run out 52-29 winners in an attacking rugby exhibition.
South Canterbury led 35-10 at halftime, with speedy winger Willie Wright on the end of two fine team tries as the Cantabrians ran the ball from their own half and Horowhenua-Kapiti dropped off tackles.
Not giving up, Horowhenua-Kapiti earned a bonus point with three late tries as both teams threw caution to the wind on the professionally manicured surface, assisted by the fact South Canterbury had a player sinbinned after a series of team infringements.
Results, Week 3.
Thames Valley 43 (Ethan Seymour 2, Matiu Abraham, Harry Lafituanai, Cameron Dromgool, Brett Ranga, Reece Broughton tries; Broughton 4 con) bt Buller 22 (Anthony Ellis, Caleb Aperahama, Anthony Tailua tries; Michael Wells pen, 2 con). HT: 19-15.
King Country 22 (scorers not provided) bt North Otago 18 (Junior Fakatoufifita, Anthony Amato, Hemaua Samasoni tries; Josh Buckham pen).
Wanganui 53 (Tyler Rogers-Holden 2, Kaveni Dabenaise, Harry Symes, Dane Whale, Craig Clare, Kameli Kuruyabaki, Shandon Scott tries; Clare pen, 4 con, Scott con). bt Poverty Bay 0. HT: 36-0.
Wairarapa Bush 26 (Sam Gammie 2, Matt Henderson, Mike Lealeav'a tries; Glenn Walters 3 con) bt East Coast 5 (Jayden Milner try). HT: 14-0.
Mid Canterbury 36 (Seta Koroitamana 2, Jarred Percival, Brian Matormusha, Dan Fransen tries; Andrew Letham pen, 4 con) bt West Coast 29 (Amenatave Tukana, Tumama Tu'ulua, Duncan Wood, Sean McClure tries; Tom Reekie pen, 3 con). HT: 21-7
South Canterbury bt Horowhenua Kapiti 52-29. HT: 35-10.
