Otago needed to win to make the championship playoffs and produced a decent performance to clinch a spot in the top four.
Impressive left winger Jona Nareki scored in the corner with an acrobatic dive after the hooter to give his side a 22-5 lead at the break.
It was far from a brilliant 40 minutes from the home side but it set up the opportunity to secure a vital four-try bonus point.
Tei Walden slipped over for that important try when he got a wonderful pass from halfback Jonathan Ruru.
Then the front row went to work and pushed Southland off its own ball to set up a try for replacement winger Latu Vaeno.
Earlier, Bay of Plenty beat Waitako 36-32 in Tauranga to nab a home semifinal.
Wellington will host the other semifinal but who it plays depends on the outcome of the game between Manawatu and Hawke's Bay in Napier tomorrow.
Manawatu can knock Otago down a slot with a good win and squeeze Northland out.
That would leave Otago with a tough away semifinal in the capital, while Manawatu would be away to Bay of Plenty.
