Harbour scored four tries to beat Southern at Watson Park in a game riddled with mistakes and wrong options by both teams.
Both teams blew try-scoring opportunities with dropped passes, knock-ons and forward passes.
When they got their act together, the Harbour backs were dangerous, with Ben Marshall controlling play effectively in the midfield.
He set up wing Iloa Lihau for his third try in the second spell and the bonus point for the team.
Lihau was elusive with the ball in hand and his two tries late in the first spell gave Harbour a 17-7 lead at the break.
No 8 Brad Griffiths, lock Graeme Cashmore and hooker Ryan Olsen played key roles in the Harbour pack that matched the strong Southern eight.
A feature of Harbour's forward play was the way the pack drove together to put pressure on Southern at the breakdown.
Powerful midfielder Luke Herden and first five-eighth Julius Glasson were the best of the Southern backs.
Hard-running flanker Carl Anderson scored Southern's only try and No 8 Simon Knight was solid and gained valuable ball at the back of the line-out.
Harbour 24 (Iloa Lihau 3, Johnny Legg tries; Craig Sneddon 2 conversions), Southern 10 (Carl Anderson try; Lewis Hancock conversion, penalty goal).
Halftime: Harbour 17-7.
Referee: Todd Pullar.
