The under 16s turned in a compelling performance to win 36 points to 10 over a strong Parramatta Eels side, and the under 18s played out a ten-all draw against the Eels as well.
The Harold Matthews side are undefeated so far this season, having won all of their nine round matches.
The SG Ball side won six matches, drew two and only lost one match on their way to a top-two finish.
The South Sydney boys will both play home Qualifying Finals at the Suttons South Sydney High Performance Centre at Redfern Oval this Saturday 13 April, with the under 16s taking on the fourth placed Newcastle Knights at 12:30pm, and the under 18s playing the third placed St George Dragons at 11am.
South Sydney 36 (Reimis Smith 2, Gabriel Hamlin, Keeden Kelly, Cameron Murray, Siosifa Talakai, Jonathan Tufuga tries; Siosifa Talakai 4 goals)
South Sydney 10 (Tulsa Saumamao, Tevita Cottrell tries; Paul Momirovski goal)
Harold Matthews Cup Final Ladder
SG Ball Cup Final Ladder