North Otago, who had a share of the lead, were beaten by the improving Wanganui side in Oamaru 22-27.
Poverty Bay were on the verge of a possible win over Horowhenua Kapiti in Waikanae after a storming final quarter. They were hot on attack knowing a try would secure a win when an intercept took Horowhenua Kapiti to a 10-point win.
Wairarapa Bush gained a share of third place with North Otago when beating Buller in Westport 20-17.
Competition for places in the top four are still unclear with South Canterbury and Wanganui sitting one point behind the third-placed teams.
Results: Mitre 10 Heartland – Week 6
Buller 17 (A Paterson, L Devery, M Kaloudigibeci tries; James Lash con) Wairarapa Bush 20 (Tristan Flutey, Brock Price tries; Tim Priest 2 con, 2 pen).
East Coast 19 (Scorers not provided) West Coast 21 (Amenatave Tukana, Steven Soper tries; Penalty try; Sean McClure 2 con). HT:
Horowhenua Kapiti 38 (Jono Ihaka 2, Jordan Tupai-Ui, Pakai Turia, Himiona Henara, Dylan Taylor tries, Henare 4 con) Poverty Bay 28 (Quade Tapsell, M McGuire, Ethan Reeves tries; Penalty try; Kelvin Smith 3 con). HT:
King Country 19 (Josh Balme 2, Zayne Tipping tries; Evan Reihana 2 con) South Canterbury 21 (Shane Anderson, Siu Kakala, Joel Smith tries; Willie Wright 3 con). HT: 0-14
Mid Canterbury 17 (Seta Koroitamana, Timoci Nabakeke tries; Tyler Blackburn 2 con, pen) Thames Valley 31 (Jason Laurich, Harry Lafituanai, Laulea Mau, Reece Broughton tries; Broughton 4 con, pen). HT: 10-14
North Otago 22 (Taina Tamou, Ralph Darling, Anthony Arty tries; Robbie Smith con, pen; Lemi Masoe con) Wanganui 27 (Lindsay Horrocks, Campbell Hart, Wiremu Cottrell, Vereniki Tikoisolomone tries; Dane Whale 2 con, pen). HT: 10-20