Harbour disposed of Zingari-Richmond 48-12 and climbed one spot into second place.
Taieri upset Dunedin 21-15 to win the Challenge Shield, while Kaikorai edged Alhambra-Union 26-25.
Southern: 34
University: 15
Southern swept aside University 34-15 to reclaim the Cavanagh Memorial at Bathgate Park on Saturday.
The Magpies dominated the set pieces and that gave powerful No8 Mika Mafi the perfect base from which to launch.
The hard-working loose forward produced a stunning opening 40 minutes in which he scored two tries and set up another.
University had all the territory early and popped over a penalty.
But the game suddenly changed when Mafi bumped off a defender and ran the best part of 40m before being held up over the line in a desperate tackle. Southern worked a move from the 5m scrum and winger Ben Leggett went over.
Mafi's line break provided the spark the home team needed and it scored twice more before halftime. Mafi rumbled over from the base of a scrum, and again when he got nice and low and dived over from short range.
There was nothing one-dimensional about Southern's next try. The Magpies swung the ball the width of the field with crisp precision.
University appeared to have the move covered but fullback Josh Buchan committed the last defender by delaying his pass to Leggett by a split second.
The winger still had a little work to do on a 40m run to the line. He swerved infield and then accelerated to beat the cover defender on the outside.
University scored two consolation tries in the final quarter of the game but its chances of making the playoffs look pretty slim.
Southern, however, is 20 points ahead of the fifth-placed side and can start thinking about how it can carry its form into the semifinals.
Harbour: 48
Zingari-Richmond: 12
Quick-thinking Harbour wing Sala Halaleva capitalised on Zingari-Richmond mistakes and used his speed to score three first-half tries at Watson Park.
That gave Harbour a 31-7 lead at the break and it added three more tries in the second spell to beat the Colours 48-12 and retain the Scoles Memorial Trophy.
The Harbour backs and supporting loose forwards had superior pace and tested the Zingari defence in the difficult underfoot conditions.
Logan Allen relished his new role at first five-eighth. He directed play efficiently and remained in support of the ball carrier to score two second-half tries.
His long touch-finders frustrated Zingari when it was in attacking positions. He landed six goals from eight attempts to score 23 of his team's points.
Hooker Sekonaia Pole was a workhorse at the breakdown and lock Alan Burling was solid in the tight. The support play of the loose-forward trio of Sine Misiloi, James Tomkinson and Charles Elton tested the Zingari defence and the strong Harbour pack pushed Zingari back at three scrums.
Zingari tightened its defence, made fewer mistakes and became more competitive in the second spell. Its experienced forwards set the example.
Flanker Tom Rowe made six clean lineout takes and was the best Zingari forward. He was backed by hard-tackling veteran No8 Chris Bell and tighthead prop Chris Preddy.
First five-eighth Thomas Johnson, the best Zingari back, had limited opportunities on attack because of the rushed Harbour defence.
Kaikorai: 26
Alhambra-Union: 25
Kaikorai hooker James Hadfield had two very relieved parents on the sideline at Bishopscourt on Saturday when the defending champion escaped with a fortuitous 26-25 victory over Alhambra-Union.
Lester and Diana Hadfield made a trip out from England to take in some early matches on the Lions tour and watch youngest son James play club rugby for Kaikorai before returning home after a sightseeing tour of the South next Sunday.
It has been an eventful two weeks for the couple so far who have watched James play the past two Saturdays, and received news that his older brother, Richard, a member of the musical theatre group, Collabro, which won Britain's Got Talent in 2014, broke an ankle in the final of the Bristol Sevens.
They were thrilled on Saturday as they witnessed James score two first-half tries in assisting the Demons to a 26-10 halftime lead.
Despite succumbing to a superior scrum, the general play from the Kaikorai forward pack gave it some respectability. Flanker Patrick McCurran, with six clean lineout takes, was faultless in defence and support play.
The Kaikorai tight five of Hadfield, Jonah Aoina, George Hendry, Cian Romaine and Pita Sinamoni featured from broken play.
For Alhambra-Union it was another game of so near, yet so far, and but for the want of a goal-kicker, Kaikorai could well have lost in centre Danny van der Voort's 100th premier game.
The Broncos' loose-forward trio of Ben Tuiomanufili, Jacob Coghlan and Ty Pelasio, in his last game for the side, worked tirelessly in shutting down the Kaikorai attack, and played a major role in Alhambra-Union's resurgence in the second half.
Taieri: 21
Dunedin: 15
Taieri won this game in the first half with an organised, spirited performance.
It put 18 points on the board and effectively that was the match.
The Eels started well and dominated territory and possession. The game's best player by quite some distance was halfback Kurt Hammer, who spotted a gaping hole after three minutes and put Kori Rupene over out wide for an early five-point lead.
The sides traded penalties midway through the spell and it took Taieri until late in the first spell to score its second try. It attacked the Dunedin line until Hammer breached the line to give his team a 18-3 lead at the break.
The Dunedin backline was simply awful in the first spell, with numerous knock-ons, aimless kicking and no clear game plan. It seemed to have learnt nothing from the previous week against Harbour.
In the second spell, the Dunedin pack broke its spell of lethargy to dominate the scrums, rucks and mauls and the pick and go. The Sharks got themselves back into the game as winger Mone Samate-Palu and replacement fullback Cameron Hay scored from broken play.
Unbelievably, at six points down with minutes to play, Dunedin had a chance to win the game as its big pack hammered away at the Taieri line. It looked only a matter of time before it scored when, inexplicably, replacement halfback Shelford Meads went the wrong way from a ruck, straight into the arms of three Taieri defenders. A maul was called and the game was done.
Taieri had played itself to a standstill and it would have been an injustice had it not won.
Hammer was outstanding around the scrum and ruck and his passing and kicking game were exemplary. The pack to a man played itself to a standstill and loosies Riley McDowall, Josh Brown and Nick Henderson tackled everybody in sight.
The wheels are falling off Dunedin's campaign after two consecutive losses and it desperately needs to find some backline leaders.
Hame Toma, Mark Grieve Dunn and Don Brighouse were its best, as usual. Highlander Gareth Evans came on for a 15-minute cameo but Dunedin really needed him on for longer.
The team will also need to get a fit hooker back on the paddock as its top three top rakes are all crocked.
