Andrew Stephens, Michael Lealava'a and Iliesa Ravudra all scored two tries for the visitors.
South Canterbury have East Coast in the final round and will be supremely confident of locking down top spot, after they had to struggle to overcome a game West Coast in Greymouth, 45-38.
The Coasters were leading 38-31 before being run down, as South Canterbury's goalkicking halfback Willie Wright had a brilliant afternoon with a 30-point haul including a hat trick of tries.
Kalavini Leatigaga scored a double to tie with Mid Canterbury's Maleli Sau for seven tries this season, although now they both trail Wanganui's Timoci Seruwalu, as his two tries on Cooks Gardens took him to eight for the campaign.
In a rarity for the Heartland Championship, there is now a significant gap from those teams currently in the Lochore Cup group to those in the bottom four who could see their season end in a fortnight - as usually one or two squads have a late chance of sneaking into the playoffs.
Poverty Bay trail eighth-placed West Coast by six points after their 34-12 loss to North Otago, who climbed two places to sixth after the win in Oamaru.
The home side scored two tries in each half to get their bonus point, all converted by Josh Buchan.
Wanganui's win on Saturday temporarily had them at third spot on the points table, given Horowhenua Kapiti had to wait to play Wairarapa Bush until yesterday afternoon in the now-annual derby game at Wellington's Westpac Stadium.
The match was the curtain raiser to the Wellington vs Otago Mitre 10 Cup clash, and as such was able to be broadcast live on Sky TV, although in the ratings stakes it was very much playing second-fiddle to the live All Blacks vs Argentina test from Buenos Aires.
Horowhenua-Kapiti led 18-0 at halftime as a yellow-card to Eddie Cranston just before the break proved costly for Wairarapa Bush, then a scoring burst just after resumption saw them go down 33-0.
A rally allowed them to score two consolation tries, before Horowhenua-Kapiti had the final say with Nathan Kendrick's second try near fulltime after Cyrus Baker had also been sinbinned.
Tyson Maki likewise scored two tries for Horowhenua-Kapiti as they maintained second spot on the points table, setting up the vital clash with Wanganui this weekend in Levin.
Results, Round 6.
Buller 54 (Andrew Stephens 2, Michael Lealava'a 2, Iliesa Ravudra 2, James Lash, Kahu Parata tries; Lash 7 con) bt East Coast 17 (Neihana Ratahi-Brown, Kris Palmer, Sailosi Vatubua; Kris Palmer con). HT: 35-7.
North Otago 34 (Petelo Pouhila, Matthew Vocea, Aaron Martin, Junior Fakatoufifita tries; Josh Buchan pen, 4 con; Lemi Masoe pen) bt Poverty Bay 12 (Everard Reid, Ethine Reeves tries; Andrew Tauatevalu con). HT: 24-0.
South Canterbury 45 (Willie Wright 3, Kalavini Leatigaga 2, Vatiliai Tora tries; Wright pen, 6 con) bt West Coast 38 (Peter Te Rakau, Josh Manning, Daniel Davis, Regan Stanton, Logan Heath tries; Nic Cumming pen, 5 con). HT: 24-21.
Mid Canterbury 25 (Jarred Percival, Willie McGoon, Christian Vainerere tries; Percival 2 pen, 2 con) bt Thames Valley 16 (Alex Bradley try; Chris Doak con, 3 pen). HT: 15-9.
Wanganui 80 (Jona Sawailau 2, Cody Hemi 2, Timoci Seruwalu 2, Cameron Crowley 2, Angus Middleton, Lindsay Horrocks, Cole Baldwin, Jamie Hughes, Nick Harding tries; Craig Clare 4 con, pen; Harding 2 con) bt King Country 3 (Kieron Rollinson pen). HT: 36-3.
Horowhenua Kapiti 38 (Nathan Kendrick 2, Tyson Maki 2, Willia Paia'aua tries; James So'oialo 3 pen, 2 con) bt Wairarapa Bush 12 (Chris Raymond, Cyrus Baker tries; Tim Priest con). HT: 18-0.