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Southern has the maximum 20 points after four games and tops the table, two points ahead of Harbour, the only other unbeaten side in the competition.
Southern, though, has played the three bottom sides in the competition, Alhambra-Union, Dunedin and Zingari-Richmond, and its other win was over University in the first game of the season.
Southern coach Luke Herden said the team had probably not played the strongest sides but he was really happy with its play.
"The boys have been playing really well. The defensive intensity from the guys has been really impressive. Defence is about attitude and desire and the guys have been working hard on that facet of the game," he said.
The forward pack has been dominant and the scrum has looked good. Otago tighthead Hisa Sasagi injured his knee in the match against Dunedin but is expected to be back in a week or two. No8 Mika Mafi missed the game against Dunedin but came off the bench in the win over Alhambra-Union last week and is looking to have a big year.
With Sam Pene developing well as a hooker and young front-rower Kurt Whittaker-Prendiville a promising player, there is plenty of depth in the Southern front row.
The backline is benefiting from new recruit Dominic Clapcott, who is keeping the side on the front foot from first five-eighth and has an educated left boot.
Outside him, the backs are running hard and straight and between them have a good mix of speed, footwork and nice hands.
Mackenzie Haugh is scoring tries in the backline and can snaffle a chance when it comes along.
Herden said Haugh had all the attributes of a key player and was a good voice on the field.
They were not sure what to expect from Green Island. Its young team was looking to build up from what it had done in the past few years.
He was looking forward to the battle between Mafi and Green Island and Otago No8 Dylan Nel.
Green Island has lost to Taieri but won its other three games and lies fourth on the table.
Green Island won the equivalent game last year in an upset but Southern will be more than ready for the men from the suburbs. The match is for the Selwyn Inglis Memorial Trophy.
In other matches, Harbour will play host to University at Watson Park, which will be an intriguing match. University was slow out of the blocks, losing its first two games, but had a convincing win over Taieri in its last match. Harbour had the bye last round.
Taieri will host Zingari-Richmond at Peter Johnstone Park, while Alhambra-Union, at the bottom of the table, hosts Kaikorai at the North Ground.