After starting the season with four losses the Stags got on the board in the crossover encounter as Hawkes Bay's Premiership woes continued and they look a likely contender to be relegated to the Championship.
Magpies' prop Adrian Barone got the first try of the match before Southland front-rowers Jamie Mackintosh and Jason Rutledge each dotted down to continue the try-scoring day for men with low numbers on their backs.
Hawkes Bay flanker Karl Lowe came close to getting the Magpies back in the game with 15 minutes left but he lost control of the ball with the line at his mercy after some great build-up work from Gillies Kaka and Andrew Horrell.
Kaka was dangerous throughout the game and was hard to contain with ball in hand but it was a disappointing outing for the Magpies who have now lost four from five this year.
When wing Tim Cornforth scored in the 69th minute Southland's four points were locked up and despite Hawkes Bay's best endeavours they struggled to find a way back in to the game.
Replacement flanker Dion Bates also crossed in the 75th minute, which gave the home side a rare bonus point.
Cowan enjoyed a busy afternoon and capped a solid outing with a try-saving tackle to deny Hawkes Bay No 8 Mike Coman in the final play of the game.
Southland's victory also leaves Harbour as the only team who are yet to have won a game this season.
Hawkes Bay host Bay of Plenty in Napier on Friday in what looms as a must-win game for both sides as they look to avoid relegation, while Southland travel to Harbour on Saturday.
Southland 35 (Jamie Mackintosh, Jason Rutledge, Tim Cornforth, Dion Bates tries; Scott Eade 3 pen, con, Marty McKenzie 2 con), Hawkes Bay 7 (Adrian Barone try; Dan Waenga con) HT: 9-7
