The Stags had struggled to get much going in the first half, though prop Morgan Mitchell showed some dash to score two tries.
North Otago defended well and fronted with a superb set piece to keep its higher-level opposition frustrated for the first 60 minutes.
Southland then lifted its intensity and pace, and both centre Aleki Morris and fullback Tauasosi Tuimavave scored braces of tries to blow out the score.