Southern (55 points) is locked in following its 38-13 win against Zingari-Richmond.
Harbour (49 points) can plan for the playoffs following its 23-10 win against Taieri, and Dunedin (47 points) also confirmed its place with its 17-5 win against Alhambra-Union.
Kaikorai (42 points) had the bye this week and can still be overtaken by Alhambra-Union (30 points).
But University is out of contention following its surprise 40-37 loss to Green Island.
Dunedin: 17
Alhambra-Union: 5
If you love the sound of the whistle and watching big forwards rumbling the ball up a metre at a time, then the North Ground was the place to be.
Dunedin's 17-5 win against Alhambra-Union did not provide much material for the highlights reel, but the victory lifted the Sharks to 47 points and secured them a semifinal appearance.
Alhambra-Union (30 points) can still sneak into the playoffs if it wins its remaining three matches and Kaikorai (42 points) loses its.
That scenario seems pretty far-fetched, though. Kaikorai has not lost since round six and has some very winnable games coming up.
Alhambra has played some nice rugby this season but Saturday's effort was lacklustre.
Dunedin controlled possession and territory. Whenever Alhambra did get the ball, it made a mistake or gave up a penalty in a game dominated by the whistle.
Dunedin battered away before Hame Toma turned all the pressure into points. He busted through the tackles of Alex Frood and Ben Tuiomanufili to score and give Dunedin a 7-0 lead at the break.
Halfback Will Cosgriff found a gap early in the second spell and passed inside to the hard-running Leroy van Dam.
He got collared in a crunching tackle by Vilimoni Koroi, but momentum carried him towards the goal line and he grounded the ball against the base of the post.
Alhambra finally got some ball in the last 10 minutes and Koroi scored a consolation try after burly No8 Ty Pelasio bounced off a defender to break the line.
Harbour: 23
Taieri: 10
Taieri drafted in the big guns, as Blues fullback Michael Collins and Highlanders Aki Seiuli and Guy Millar joined the team for the Challenge Shield defence against Harbour.
Despite the extra help, the Eels produced an error-ridden display at Peter Johnstone Park in what was its most important game of the season.
With its chances of making the semifinals gone, the team was hoping to lock up the Challenge Shield for the rest of the season in its last home game.
But Harbour pounced on Taieri's mistakes, scoring late in the first half to take a 11-10 lead into the break.
The ball was swung wide from a lineout and winger Hemaua Samasoni went over in the corner.
A few minutes earlier, Taieri scored the game's opening try when No 8 Riley McDowall, who was at the back of a lineout drive, muscled his way to the chalk.
Harbour stretched its lead to 18-10 when Ewan Brumwell scored out wide and Hawks first five-eighth Logan Allen drilled the conversion.
The visiting team attacked the try line late in the game and lock Chucky Koroi scored to complete the 13-point win.
The victory confirmed Harbour's place in the semifinals.
Taieri made too many mistakes, particularly in the second half.
For Harbour, Allen had a cracking game and his tactical kicking was spot on.
Loosie Ben Whale was the standout Harbour player in the pack, while Seiuli was good for Taieri and halfback Kurt Hammer produced another solid performance.
Green Island: 40
University: 37
Playing his first game of the season for Green Island, Highlanders prop Siate Tokolahi dived from a maul to score in the final minutes and seal a 40-37 victory over University at the University Oval.
With both sides playing an expansive game, it was University that stole an early advantage when right wing Tom Morrison scored twice within the first five minutes.
It proved an early wake-up call for Green Island, which regrouped and replied with two tries of its own. But lapses from the visiting side allowed University to capitalise and leap out to a 29-14 lead.
The final minutes of the half led to a scoring spree - Green Island scored two further tries and University its fifth enabling both sides to record four-try bonus points by halftime.
From the restart, Green Island continued to dominate possession and scored early, to bring it back within range of University's first half tally of 34 points.
Defensively, Green Island took its ''A'' game on the field with it in the second half. Out wide, wingers Alex Barnsley and Brad Tunnicliffe were impenetrable. They were well supported by team captain Hayden McBride, along with flankers Matt McCutcheon and Tim Heller. Lock Jared Williams proved a driving force in broken play and at the lineout. Williams was a key player in Green Island's maul that led to Tokolahi's try to win the game.
Openside flanker Sam Dickson proved key for the University pack and found it plenty of space with his surging runs. Locks Damien Scott and Sione Teu featured in a pack that was forced to work hard against a side ready to make the most of every opportunity.
Southern: 38
Zingari-Richmond: 13
A dominant Southern pack controlled set pieces in the first spell and ground an injury-hit Zingari-Richmond into the ground in the second spell at Montecillo.
Zingari had a territorial advantage in the first spell but a late try by No8 Mika Mafi gave the Magpies a 17-8 advantage at the break.
Determined Zingari tackling had shut down Southern's scoring chances in the first half but a tired Colours defence allowed Southern to open up its game in the second spell.
Flankers Axel Hohneck and Harry Flood gave Southern a two-to-one lineout advantage. Three of Southern's five tries came from lineouts.
The front row of Craig Millar, Joe Weatherall and Mike Mata'afa and lock Simon Knight gave Southern a dominant scrum and three times pushed Zingari off the ball on three.
Mafi's decisive try late in the first spell came from a powerful scrum that gave him space to run 30m to the line. It was his 10th for the season.
Josh Walden was an efficient halfback and his long passes gave first five-eighth Josh Ioane space to weave his magic and score an early second-half try that changed the momentum of the game.
Ioane kicked six goals from six attempts and scored 18 points to take his season's tally to 149.
Fullback Josh Buchan was dangerous on the counterattack and used his pace to join the line and score two second-half tries.
The best Zingari forward was No8 Chris Bell with his tireless defence. He was backed by hooker Greg Pleasants-Tate and prop Chris Preddy.