The 2018 Southern Football League was the first season of the second level of men's football in New Zealand's South Island.
Caversham were the 2017 South Island Football Championship champions. Ferrymead Bays were the 2017 Mainland Premier League champions and Caversham were the 2017 FootballSouth Premier League champions.
The 2018 Mainland Premier League was the 17th season of the league, the 21st season of premiership football in the upper South Island. The shortened season started on 17 March, and finished on 9 June, being a double round. The league was to determine the top five teams in Mainland's federation, who go on to play the top three teams from FootballSouth; with the bottom three will play in the teams with will play each other twice to determine who will face relegation to the Canterbury Championship League, both starting after this league finishes.
Eight team are competing in this league - seven from the previous season and one promoted team. The promoted team is Selwyn United, who replaced Western after five seasons.
<onlyinclude></onlyinclude>
The 2018 FootballSouth Premier League was the 19th season of premiership football in the lower South Island. The shortened season started on 7 April, and finished on 16 June, being a single round. The league was to determine the top three teams in FootballSouth's federation, who go on to play the top five teams from Mainland Football. The bottom seven team will play in the in FootballSouth Championship League, starting after this league finishes.
Nine teams are competing in the league - all teams from the previous season.
<onlyinclude></onlyinclude>
The Southern Football League is the culmination of the two regional leagues, each of which provide teams to the Southern Football League. The top five teams in the Mainland Premier League and top three teams from the FootballSouth Premier League qualify for entry into the Southern Football League.
<onlyinclude></onlyinclude>