Supporters from both sides turned up in numbers to the Murray Barracks oval to watch what was the main game of the day played in dry and windy conditions.
Flies took a 4-0 lead after only 9 minutes and extended this to 10-2 at half time after a 90 metre try from Gabby Taune.
The second 40 minutes of play belonged to Souths who held the Flies scoreless.
A penalty and a try put the score at 10-8 with 24 minutes left on the clock.
10 minutes later, Souths took the lead after a try from prop Kingston Kambi.
Halfback McKenzie Buka took his time and slotted the ball through the posts to give Souths a four point lead.
Souths then defended their narrow lead to the finale whistle to end the Flies season.
In other results from the qualifying finals, the U19 men's games saw Paga Panthers beat Butterflies 6-0 while West defeated Royals 4-2 in the other game.
In the other A Grade men's game, Kone Tigers came out victors beating the Dobo Warriors 8-4.
The women's game between Kone Storms and Paga Panthers was a one sided affair with the Panthers winning 34-0.