Southern slipped back to second following an 18-12 loss to a tenacious Harbour side.
Kaikorai secured its fourth win in a row with a 27-7 victory against Zingari-Richmond while University held off Taieri to win 22-14.
Harbour: 18
Southern: 12
Harbour will take heart from a gutsy second-half defensive effort in which it held Southern to just three points in miserable conditions at Bathgate Park.
The Hawks led 18-9 at the break and had to spend long periods camped in their own territory defending their line.
The task got even tougher when the referee ran out of patience and sent first five-eighth Rory Parata to the bin following a series of infringements.
But for the most part Harbour maintained its discipline and produced some stout defence.
The loose forward trio of James Tomkinson, Ben Whale and Sione Misiloi earned their hot showers. No-one knows exactly how much mud they carried in there with them but it was plenty.
Harbour's tackle count mounted quickly and so did the handling errors for both sides. It was not a good day to be a back but Harbour did display some quality handling in the second of its two first-half tries.
Harbour forced a turnover about 25m out. The ball was swung left and went through a couple of sets of hands before Ewan Brumwell got the ball and dashed towards the corner.
A little earlier, lock Alan Burling scored from close range after his side had been battering away at the line.
Southern replied with a late penalty to trail 18-9 at the break. The second half turned into a grind and Harbour was winning the battle through tenacious defence.
With about a minute remaining, Southern got a penalty and opted to kick at goal and salvage a bonus point rather than roll the dice and kick for touch.
University: 22
Taieri: 14
University held off a resurgent Taieri to secure a 22-14 win under lights on the University Oval No 6 ground.
Despite the 2.45pm kick off, the conditions were bad enough that lighting became necessary.
It is a rare occurrence indeed for premier rugby in Dunedin.
University began the match strongly, making good use of a strong wind at its back to dominate territory. But such was the commitment from the Taieri defenders, University's two tries in the first half were scored in the final six minutes before the break, each coming somewhat out of the blue and with a touch of individual skill.
Left wing Tom Morrison had to elude several Taieri defenders on a 20m run to score out wide. Then, on the stroke of halftime, first five-eighth Fletcher Smith ducked and weaved to wrong-foot the Taieri inside defence and score under the bar.
Both tries complemented the superb service and option-taking skills of halfback Connor McLeod, who has all the hallmarks to add his name to the long list of great halfbacks to have come out of the University club.
McLeod provided the genius - which was in the end the difference between the two sides - when he chip kicked ahead, chased and applied pressure to three Taieri defenders on their own line to force a 5m scrum. The resulting tighthead led to a try to prop Angus Williams.
McLeod's passing was crisp and accurate throughout, and his option taking was excellent, which made for an interesting contest when Ryan Hammer came on at halfback for Taieri for the final 30 minutes.
Dunedin: 40
Green Island: 5
Dunedin played great wet-weather rugby in the first spell with a brisk southerly at its backs and led by 27 points at the break. Conversely, Green Island did not play great-wet weather rugby with the wind at its back in the second spell and it consequently lost by 35 points.
Sharks first five-eighth Sharks Louis Tili used the breeze effectively in the first spell allowing Dunedin to totally dominate territory and possession in the greasy waterlogged conditions and the the points followed.
Its pick-and-go game was ideal for the inclement conditions and resulted in three tries, Captain and lock Mark Grieve Dunn had one and prop Don Brighouse scored a brace from these surges. Winger Zac Harrison Jones put the icing on the cake when fast hands put him over out wide. With a 27-point lead and a bonus point in the bag, it looked comfortable for Dunedin at the break.
Green Island turned with the strong breeze but inexplicably never kicked for position, instead trying to run the ball. Its handling in the backs was substandard and Dunedin constantly had it under pressure.
Astonishingly, Dunedin had the better of the territory in the second spell. Tim Cossens kicked two handy penalties and replacement prop Teague McIlroy crashed over for Dunedin to hit 40 and go top of the competition.
Green Island finally got on the board late in the game when midfielder Hayden McBride dashed round the short side of a ruck to score a consolation five-pointer.
For Dunedin, Louis Tili was commanding at first five and Jamie Mowat, his nose glued to the ball, was indefatigable in the loose. Harrison Jones looked sharp out wide with his limited opportunities. The other loosies, Profitu Fesili and Hame Toma, were powerful with the ball in hand. Big Don Brighouse was a powerhouse on the pick and go.
For Green Island, No8 Jared Williams, lock Woody Kirkwood and hooker Brett Kingsbury were tireless on defence in a beaten pack.
Kaikorai: 27
Zingari-Richmond: 7
A dropped ball on the Kaikorai line midway through the first spell proved costly for Zingari-Richmond and was the turning point in a hard-fought game at Bishopscourt.
The hard-driving Zingari forwards were poised to score their second try close to the posts to give their side a 14-5 lead, when playing into the strong southerly.
Instead, the ball was snapped up by the Kaikorai backs who ran it 40m and then put in a kick wing Jordan McEntee gathered for the 100m turnaround try.
At that time, Eli Tonga and Tom Rowe were dominating the lineout and Jack Wolfreys and Chris Bell were tigerish in the loose.
The try snapped the growing confidence of the Zingari forwards as Kaikorai scored three first-half tries with the wind to lead 17-7 at the break.
Powerful prop Jonah Aoina led a dominant Kaikorai scrum and the pack took control to shut Zingari out in the second spell.
Lock Cian Romaine was difficult to stop close to the line and flankers Isimeli Tuivaga and Lakopo Petelo Mapu were solid on defence and lethal on attack.
Kaikorai rushed up on defence and made it difficult for the Zingari backs to break the advantage line. The probing short-side runs and tactical kicking of halfback Josh Renton opened up attacking opportunities.
Second half tries to Tuivaga and Chris Whitley gave Kaikorai a comfortable win in the atrocious conditions.
