Reigning champion Caversham plays Western at Tonga Park, while Woolston Technical hosts Dunedin Technical at Garrick Park in Christchurch.
Both semifinals kick off at 2pm, with extra time and penalties being used, if required, to find the winners.
If at least one Christchurch side wins, the final will be held at English Park on September 19.
If both Dunedin clubs win, the final will be at the Caledonian Ground on September 12.
Caversham coach Steve Fleming said having to play on Tonga Park - because the Caledonian had been booked by athletics - was unfortunate as the surface was "far from ideal".
As it turned out, both Caversham and Western did not have games last weekend and could have played then.
Neither Dunedin side has had good preparation for the semifinals, as Footballsouth competitions have tapered off in the aptly named end-of-season tournament.
Whether the Christchurch sides have had better preparation remains to be seen, although the fact Western was knocked out of the English Cup 3-0 by Nomads suggests greater depth of competition in Canterbury.
Apart from not playing for three weeks, Caversham's squad has shrunk from 18 to 12 players, and whether striker Tom Jackson will play still depends on his plans to leave for the United States, where he hopes to play for a college.
Caversham won the inaugural South Island title last year, beating fellow Dunedin side Roslyn-Wakari 6-1 in the final.
Caversham wrapped up its sixth successive Footballsouth Premier League title this year, its ninth championship since 2000.
In three of the past four seasons Caversham has also reached the last eight of the Chatham Cup, twice going on to reach the semifinals under Fleming's guidance.
Dunedin Technical finished three points behind Caversham, with 13 wins and three losses, and Golden Boot winner Aaron Burgess scored an impressive 32 of his side's 56 league goals.
Coach Mike Fridge's side left yesterday by bus for Christchurch, but travelled without several key players.
Ross McKenzie and Justin Flaws have left Dunedin, and striker Alistair Rickerby has work commitments.
"We still have a strong squad capable of getting the job done, and hopefully earning a place in the final," Fridge said.
"We have had reports on Woolston's style of play, but my feeling is to let them worry about us.
"We still have a quality nucleus in Aaron Burgess, Andy Coburn, George Milne and Blair Scoullar, plus a strong youth contingent that has graduated through several Milk Cup campaigns."
In the Mainlands league, Woolston Technical finished five points ahead of second-placed Western, with Burnside third and Nomads trailing in fourth place.
