The backs scored five of the six tries, but a powerful effort from the tight five helped set up the win.
Otago’s scrum has been impressive this season. Midway through the second half the home side shunted Southland back and snaffled a tighthead.
Halfback Josh Renton swung the ball wide to winger Latu Vaeno, who fended off a tackle and ran in for a try.
Some lights had blinked off moments before and a loud and sinister thunder clap had echoed around a dimly lit stadium.
Vaeno’s try had condemned Southland (0-10) to a winless season for the first time since 1990.
The visitor scored a consolation try before replacement Otago prop Jonah Aoina completed the scoring when he crashed over from the base of a ruck late in the match.
A broad smile enveloped Otago lock Josh Dickson’s face when he talked the Otago Daily Times through the side’s scrum shortly after the final whistle.
"We put a lot of effort into winning that scrum contest, and having a 100% success [on our own ball] obviously means we’re going well," he said.
"We’ve just got to make sure we keep it at 100% next week."
It was Southland who got the early traction, though. It employed a clever kicking game to control territory.
Elusive fullback Lewis Ormond, who scored two tries in the game, slipped a pass out of the back of his hand to winger Ryan Tongia, who chipped ahead.
There was a scramble in the in-goal area and the TMO was called on to adjudicate. Otago escaped with a re-start from the 22, but was almost caught napping.
The opening points came from a 5m scrum. Winger Mitchell Scott injected himself at first receiver and brushed off the tackle of halfback Jay Renton to score.
Josh Ioane added a penalty, before Southland responded with a try in the corner to Ormond.
Otago had stumbled through the opening stanza, but finished the half strongly with two nice tries to take a 22-5 lead into the break.
Left winger Jona Nareki grabbed his ninth try for the season with an acrobatic effort in the corner after the hooter.
He got a nice pass from Tei Walden, but the play was set up by another big shove by the Otago scrum.
Otago halfback Jonathan Ruru made the pass of the match, though. Ten minutes into the second half he fired the ball to Walden, who had run a wonderful angle and went over from 5m out.
It was the bonus point try and gave Otago a healthy 29-5 lead.The bonus-point win meant Otago was able to leapfrog Northland and cement a semifinal.
"We’re pretty happy. We knew we had to win this game and for other results to go our way," Dickson said.
"Southland went out there and had nothing to lose. This was their test match and we knew they were going to come out firing and we had to give it back to them."
Mitre 10 Cup
The scores
Otago 43
Mitchell Scott, Sio Tomkinson, Jona Nareki, Tei Walden, Latu Vaeno, Jonah Aoina tries; Josh Ioane 5 con, pen
Southland 19
Lewis Ormond 2, James Schrader tries; Scott Eade con, Greg Dyer con
Halftime 22-5