Setefano Sauqaqa grabbed three tries for the visitors, while Luke Harvey got a double for the hosts.
This means Wairarapa Bush's bonus point left them tied with Horowhenua-Kapiti, so they locked down fourth spot by virtue of winning the September 26 clash between the two teams.
They will now go down to Timaru for the 1st vs 4th Meads Cup semifinal with South Canterbury, who had no trouble retaining their spot on the ladder with a 62-19 mauling of a hapless Poverty Bay in Gisborne.
Erenimo Tau scored four tries for the Cantabrians, while Junior Kolisio and Paula Fifita got doubles.
So, despite maximum points from their final three games, Wanganui were still short of Mid Canterbury and barely missed a Cooks Garden's semifinal - heading instead to Ashburton for the 2nd vs 3rd clash.
There was further heartbreak down the table as Thames Valley ended West Coast's hopes of a Lochore Cup spot with a 29-17 victory, coming from behind at halftime.
The final placings were decided by King Country's Saturday night game in Taupo against also-rans East Coast, and they secured their Lochore Cup spot with a 60-14 massacre after leading by 31 points at halftime.
Horowhenua-Kapiti and Buller thus get the home Lochore Cup games.
Results, Week 8
South Canterbury 62 (Erenimo Tau 4, Junior Kolisio 2, Paula Fifita 2, Jared Trevathan, Theo Davidson tries; Trevathan 4 con, Jason Merrett 2 con) bt Poverty Bay 19 (Tamanui Hill, Kerehama Barrett, Carl Riini tries; Ethine Reeves 2 con) HT: 31-14.
Thames Valley 29 (Hayden Anderson, Josh Elgoran, Shayne Johnson, Tyrone Keith tries; Jonnie TeRuki con; David Harrison pen, 2 con) bt West Coast 17 (Jason Wright, Nik Cumming, Isei Lewaqui tries; Cumming con). HT: 17-12 West Coast.
Buller 44 (Setefano Sauqaqa 3, Taione Ratu, Revudra, McIlroy tries; Andrew Stephens 2 pen, 3 con) bt Horowhenua Kapiti 31 (Luke Harvey 2, Scott Cameron, Perry Hayman tries; Brandyn Laursen 4 con, pen). HT: 20-3.
Mid Canterbury 39 (Matt Thatcher 2, Tama Tuirirangi, Seta Koroitamana, Ashton Tuck tries; Andrew Letham 2 pen, 4 con) bt North Otago 36 (Lemi Masoe, Sam Sturgess, Sali Halaeva, Aleki Morris, Cameron Keenan tries; Ben Palmer pen, 4 con). HT: 20-10.
Wanganui 57 (Michael Nabuliwaqe, Bryn Hudson, Ace Malo, Kamipeli Latu, Samu Kubunavanua, Sam Madams, Steven Perofeta, Trinity Spooner-Neera tries; Spooner-Neera pen, 7 con) bt Wairarapa Bush 35 (Andy Humberstone 2, Andrew Makalio, Cody Whittaker, Cameron Hayton tries; Tim Priest 5 con). HT: 47-0.
King Country 60 bt East Coast 14. HT: 38-7.