The win also keeps their four-point buffer at the top of the Hilux Heartland Championship table, with one round to play before the semi-finals.
There is a clear top four and a clear next four, but that could all change with some blockbuster matches set for the final round of the regular season.
Buller pushed King Country in the first half, trailing 19-12 at the break, but the home side lifted in the second half and ran in six tries. Hone Mathieson scored a round-high 22 points with two tries and six conversions.
Mid Canterbury sit second after a 34-5 bonus-point win over West Coast in Greymouth. The match was scoreless until midway through the first half, and Mid Canterbury led 12-5 at the break. Raitube Vasurakuta scored twice for the visitors.
Thames Valley edged South Canterbury 43-42 in a tight finish, with an 80th-minute Quinn Collard penalty snatching the win for the home side.
The match swung back and forth throughout. South Canterbury scored three tries in 10 minutes late in the first half to lead 27-19 at the break, and stretched that to 34-19 before Thames Valley rallied to lead 40-39 in the 63rd minute.
Taine Cordell-Hull slotted a penalty to put the visitors back in front, but Collard had the final say. The win lifts Thames Valley into equal third with North Otago.
North Otago beat Horowhenua Kāpiti 45-31 in a high-scoring encounter. They raced out to a 31-0 lead in as many minutes, and Horowhenua Kāpiti replied with two tries before half-time. That early burst was enough for North Otago to hold on through a more even second half.
Ngāti Porou East Coast hosted neighbours Poverty Bay in the highest-scoring match of the round, with the visitors running in 11 tries in a 69-43 win. Winger Cohen Loffler scored four tries as Poverty Bay claimed their second win of the season.
Wairarapa Bush locked in their play-off spot with a 37-25 home win over Whanganui. They scored four tries to three, with the reliable boot of Hoera Stephenson adding 17 points. Wairarapa Bush will now be a genuine contender for the Lochore Cup come play-off time.
King Country have secured a home semi-final in the race for the Meads Cup, but the rest of the table is still up for grabs. Six teams are in contention for the other three Meads Cup semi-final spots, with the next four heading into the Lochore Cup.