Mid Canterbury enjoyed a 50-32 four-try bonus point win over East Coast in Ashburton which allowed them to move four points clear of Wanganui.
However, South Canterbury, with a 35-19 win over Thames Valley slipped into second place two points behind neighbours Mid Canterbury.
Horowhenua Kapiti put some points between it and the chasing bunch when it beat its nearest challenger North Otago 26-22.
Heartland results from Week Five:
West Coast 36 (Regan Stanton, Andrew Connors, Brad Houston, Nic Cumming tries; Cumming 2 con, 3 pen)
Wairarapa Bush 33 (Andrew Makalio 2, Cody Whittaker, Paul Tikomainavalu, Viliami Hala tries; Tim Priest con, 2 pen)
Wanganui 17 (Roman Tutauha, Bryn Hudson tries; Trinity Spooner-Neera 2 con, pen)
King Country 17 (Iliesa Tavuyara, Joe Perawiti tries; Zayne Tipping 2 con, pen)
Buller 36 (Peni Nabainivalu, Iliesa Ravudra, Blair McIlroy, Logan Mundy, Mitieli Kaloudigibeci tries; Andrew Stephens 4 con, pen)
Poverty Bay 14 (Tamanui Hill, Murray Hewson tries; Ethine Reeves 2 con)
Horowhenua Kapiti 26 (Brandyn Laursen, Teegan Minkley 2 tries; Laursen con, 3 pen)
North Otago 22 (Nathan Cargo, Oscar Pamment tries; Ben Palmer 4 pen)
South Canterbury 35 (Paul Fifita 2, Sam Vea, Nick Annear, Sione Vea tries; Jason Merrett con, pen; Jared Trevathan con, pen)
Thames Valley 19 (Tyrone Keith, Jonnie Te Ruki, Hayden Anderson tries; Jonnie Te Ruki con; David Harrison con)
Mid Canterbury 50 (Matt Thatcher 2, Eric Duff, Andrew Letham, Murray Williams, Gideon Lambrechts 2, Nete Caucau tries; Andrew Letham 5 con)
East Coast 32 ( Lennon Carrington, Donny Mill, Courtney Mita, Whakarae Henare tries; Kris Palmer 2 con, pen; Richard Apanui con, pen)