Almost total domination paid off for Bay of Plenty when prop Aidan Ross crashed over after a well-organised lineout drive for an 8-3 lead. Southland finally had a spell in possession with 15 phases but the Steamers defensive line was impressive.
Bay of Plenty came close from another strong set-piece platform, winning a penalty that Haimona missed with 10 minutes left in the half.
Dan Hollinshead, playing at second-five, missed another handy penalty and an easier conversion to another well-worked lineout drive finished off by No 8 Joe Tupe.
Halfback Ruki Tipuna had a fine first half sparking multiple attacks before being replaced by Highlanders half Te Aihe Toma at halftime.
Leading 13-3, Bay of Plenty were quickly under pressure with Southland crossing the line through Jimmy Cowan but were called back for obstruction.
A poor bounce and sloppy cover at the back cost the Steamers as James Schrader scored, with the conversion closing the lead to just three.
The momentum changed dramatically as those earlier missed points came back to haunt the Steamers.
Southland backed their scrum after coming close to scoring through Hawera and it worked as No 8 Elliott Dixon scored. The conversion gave the Stags 14 unanswered points and the lead 17-13.
Hollinshead kicked a penalty from 40 metres in front with 20 minutes left. Ziggy Fisi'ihoi was found by the TMO to have struck a Southland player in back play. The decision seemed harsh but Hawera missed the resulting penalty.
Bay of Plenty winger Regan Ware made a superb 50 metres break but a promising move ended with a costly breakdown penalty.
Solomona Sakalia was sinbinned with 10 minutes to go. Hawera slotted the goal for a 20-16 advantage they held on to.
In the final play Bay of Plenty had an attacking lineout 23 metres out but conceded a penalty to give Southland their first win of the season on a plate.
Southland 20 (James Schrader, Elliott Dixon tries; Wharenui Hawera 2 cons, 2 pens) Bay of Plenty 16 (Aidan Ross, Joe Tupe tries; Kelly Haimona pen; Dan Hollinshead pen) HT: 13-3 BOP