Southern adapted better to the wet and muddy conditions and played bull rush-style rugby to comfortably beat Harbour at Watson Park.
Southern kept play in the forwards, used the pick and go, and played the game in the Harbour half.
Harbour attempted to play dry-weather rugby and came unstuck, with a series of knock-ons and dropped passes that gave the advantage to Southern.
The first try was scored by winger Harry Uffindell after 17 minutes and Ben Patston added two penalty goals to give Southern a 13-0 lead at the break.
Conditions had become almost impossible for rugby in the second spell. An early penalty gave Southern the lead 16-0.
Harbour was forced to play catch-up rugby after this and found it impossible to break out in the conditions.
The Harbour defence was better in the second spell and the final Southern try did not come until the finish, when dynamic flanker TJ Ioane burst to the line from the Harbour 22m.
Ioane was the best player on the paddock with his skill on the drive and the pick and go. The other players to stand out in the Southern pack were lock Sam Gibson, No 8 Tom Franklin and openside flanker Dan Baxter.
Halfback Robbie Smith was tidy and was the best Southern back, with his probing runs down the short side and his kicks over the top.
Prop Mike Macdonald stiffened up the Harbour defence when he came on in the second spell and loose forwards Dalton Ellis and Angus Duckett had lively games.
Southern 23 (Harry Uffindell, TJ Ioane tries; Ben Patston 2 con, 3 pen), Harbour 0. Halftime: 13-0.
