The thriller was a fitting occasion for Buller's Logan Mundy to celebrate his 100th game for the union.
The loss moved Mid Canterbury out of the top four, with Lochore Cup holders King Country climbing after their second win, coming from behind to beat Poverty Bay 30-24 in Taupo and push them down to sixth.
Trailing 18-2 at the break, King Country scored all four of their tries in the second half.
North Otago moved up to fifth with a win in their first home game, holding off Thames Valley 30-22 in Oamaru.
Halfback Zayn Tipping scored a try and landed three kicks.
In the big game played down in Wellington as a curtain raiser match at Westpac Stadium, Wairarapa Bush shook off last weekend's disappointment to defeat neighbours Horowhenua-Kapiti 32-22.
Screened on Sky Sport 3, Wairarapa Bush scored four tries to two, while Horowhenua-Kapiti received the competition's first eight-pointer for a penalty try.
Having had plenty of experience playing at the stadium, former All Black Zac Guildford scored his first try in the colours of his family's home province.
Results
Wanganui 74 (Simon Dibben 2, Te Rangitira Waitokia 2, Kameli Kuruyabaki, Cole Baldwin, Tremaine Gilbert, Gavin Thornbury, Craig Clare, Ace Malo tries; Dane Whale 4 con, Steve Crosbie 3 con) bt East Coast 8 (BJ Sidney try; Todd Dolan con). HT: 36-8.
North Otago 30 (Ralph Darling, Josh Casey, Matt Posea, Lemi Masoe tries; Casey 3 con) bt Thames Valley 22 (Jonnie Te Ruki-Chambers, Rupeni Vakacugu, Hayden Wisnoski tries; Wisnoski 2 con).
Buller 34 (James Lash, Anthony Tailua, Joshua Tuidriva, Dan Hytongue tries; Lash 4 con, dropped goal) bt Mid Canterbury 32 (Logan Bonnington, Jarred Percival, Mike Lynch, Timoci Nabakeke tries; Percival 4 con). HT: 16-16
King Country 30 (Sam Mason, Kieron Robinson, Ralulu Bovau, Zayn Tipping tries; Tipping 2 con, pen) bt Poverty Bay 24 (Les Hills, Tani Misikosi, Ratu Daurua tries; Isaia Vuki 2 con, 2 pen). HT: 18-2 Poverty Bay.
South Canterbury 58 (Viliame Logavatu 2, Mosese Aho, John Dyer, Raitube Vasurakuta, Maxx Morris, Kieran Coll, Shay Moseby tries; Zac Southen 5 con) bt West Coast 20 (Troy Tauwhare, Maleli Mudu, Logan Winter tries; Nik Cumming con). HT 22-12.
Wairarapa Bush 32 (Andrew Smith, James Goodger, Zac Guildford, Sam Monaghan tries; Tim Priest 2 con, 2 pen) bt Horowhenua Kapiti 22 (Tyson Maki, penalty try tries; Perry Hayman con, 2 pen; Warwick Lahmert pen).