South Canterbury won 58-21 after leading 27-14 at the break and with the victory, it goes to the top of Heartland championship, following Mid Canterbury's upset loss to Horowhenua-Kapiti.
Next Saturday, North Otago meets Mid Canterbury in Oamaru in the final round. Five teams: South Canterbury, Mid Canterbury, Wanganui, Wairarapa-Bush and Horowhenua-Kapiti are assured of semifinals berths and North Otago, King Country, Buller and West Coast will fight it out for the final three places.
Only two points separate the teams.
At Alpine Energy Stadium on Saturday, North Otago, after an encouraging first-half, disintegrated in the second half and in the final quarter it battled, and failed to score a fourth try, which would have given it a vital bonus point.
South Canterbury skipped out of the blocks and was up by 17-0 in as many minutes and it loomed as a long afternoon for the visiting team.
But three minutes into the second quarter, North Otago finally produced some co-ordination and centre Sala Halaleva scampered in for a try, which Jason Forrest converted.
Three minutes later midfielder Aleki Morris split the defence and hooker Sam Sturgess was in for Forrest to convert and narrow the margin to three points.
Trevathan kicked a penalty five minutes later and three minutes from halftime a Sam Vea break put centre Miles Medlicott in for Trevathan to convert and give his side a 27-14 halftime lead.
North Otago coach Barry Stevens said at halftime the emphasis was placed on continuity.
''We just had to put more phases together. It doesn't matter who you are playing, if you can put together seven or eight phases, holes start to appear,'' he said.
''But when you rush that pass after only two or three phases or throw a 50-50 pass, you just put pressure back on yourselves and that's what we did.''
A Theo Davidson try early in the second half took his side out to a 32-14 lead before North Otago struck back with a try to prop Rob Mafileo, converted by Forrest.
That was it for North Otago, as South Canterbury ran away with the match.
It was a disappointing final 40 minutes for Stevens.
''Our aim in the finish was to at least get a bonus point out of it, as she's a real logjam of points in the competition and a bonus point would have put us in a better position, but South Canterbury is a well drilled, well coached side.
''They've got a good skill level. You turn the ball over as we did and they're behind your game line. But at times our individual skills execution was poor and we appeared to be rushed.''
Too often at scrum time, North Otago's defensive line was not firm.
First five-eighth Jason Forrest moved to halfback, halfback Inoke Naufahu went to fullback and fullback Lemi Masoe took up position at first five-eighth.
Stevens said it was a defensive ploy, to strengthen the defence.
Buller beat Thames Valley 25-17, Wairarapa Bush downed East Coast 45-3, Horowhenua-Kapiti recorded the upset 47-43 win over Mid Canterbury, Wanganui accounted for Poverty Bay 55-23 and King Country beat West Coast, 36-22.
