you are bang on the money about the west coast and mainland,
there is now a "southern football league which is played after the mainland and otago leagues have finished which brings together the top 5 of the mainland premier league and the top 3 of the southern premier league to find a overall south island "champion"
here is the link that covers this combined leagues teams and fixtures http://www.footballsouth.co.nz/COMPETITIONS-1/Fixtures-Results/Southern-Football-League
here is the link that can help you out with the southern premier league (contains 7 dunedin teams, 1 from queenstown and 1 from southland (invercargill) http://www.footballsouth.co.nz/COMPETITIONS-1/Fixtures-Results/Otago-Senior-Competitions
here is the link for Southland football which its topped by the "Donald Gray league" after its completion ( play each team 3 times ). the top four play knock out 1v4 2v3 to win the charity cup http://www.footballsouth.co.nz/COMPETITIONS-1/Fixtures-Results/Southern-Football-League the division below is division 1 presidents cup (again everyone plays 3 times) divsion 2 patron cup plays a a sinlge round of round robin and the splits top 6 into division 2 cup and bottom 4 into the division 2 trophy.
if you decide to do southland i can help you with club colours , grounds etc, i lived down there until i was 21 and have strong family ties to Queens Park in Invercargill ( dad has a memorial trophy named after him) so have a few contacts to dig out info