Southern fullback Joe Hill used his express speed to break the hearts of Harbour supporters and help the Magpies retain the Speight's Challenge Shield at Bathgate Park.
Harbour dominated the first half but butchered two scoring opportunities with 50-50 passes and inept handling.
The strong Southern defence held tight and when a scoring opportunity came, Hill raced down the blindside to score the try to give Southern a 13-6 lead at the break.
Hill added another try after 25min in the second spell when he broke out of his own 22m.
The movement passed through 10 hands before Hill was up in support to score the try.
Harbour had its chances and remained close but it was guilty of too many mistakes that let Southern off the hook.
Southern centre Apelu Alapati was dynamic with the ball in hand and was supported in the midfield by Luke Herden, who was a rock on defence.
Lock Sam Gibson was effective in the lineout and flanker TJ Ioane was superb in the loose with his ability to grab ball at the breakdown and break tackles.
Ben Patston kicked five goals and scored 13 points.
Harbour halfback Johnny Legg was dangerous when running down the short side and flankers Hadley May and Angus Duckett worked hard to tidy up Harbour mistakes.
Southern 28 (Joe Hill 2, Hamish Buick tries; Ben Patston 2 con, 3 pen), Harbour 19 (Ollie Cuthill, Damien Keen tries; Cuthill 3 pen). Halftime: 13-6.
