Flanker Eben Joubert played an inspiring game to help Harbour upset Southern at Bathgate Park.
Joubert dominated the back of the line-out, was a terrier in the tight-loose and scored the first try after five minutes to give Harbour the initiative that it never surrendered.
He formed a tidy loose forward combination with Jason Kelly that did not give the Southern backline any freedom.
Dan Snee played an inspired game at first five-eighth and the midfield combination of Aaron Bancroft and Siaosi Folau was an effective attacking unit.
The Harbour backs were prepared to attack from all parts of the field and the best try of the game came after 35 minutes when Harbour attacked from under its own posts.
Bancroft and Kelly combined in a passing rush that led to a try by wing Joel d' Sousa to give Harbour a 13-5 lead at the break.
Harbour continued in the same attacking mode in the second spell to score 10 more points.
The best Southern players were veteran prop Warren Moffat, flanker Ollie Spence and midfield back Luke Herden.
Harbour 23 (Eben Joubert, Brad Griffiths, Joel d'Sousa tries; Johnny Hughes conversion, 2 penalty goals), Southern 5 (Julius Glasson try).
Half-time: Harbour 13-5.
Referee: Tim Baker.
Harbour 23
Southern 5
