Green Island applied pressure from the start and gave the more fancied Southern a fright with a draw at Miller Park.
Green Island displayed its mettle from the start when rampaging No 8 Nasi Manu burst from a scrum to give a warning to the defence.
Manu was able to break the advantage line at will to keep the initiative with Green Island.
Another telling point came when new first five-eighth Alex Davies kicked a long-range penalty goal and narrowly missed another one from halfway three minutes later.
Davies had a decisive influence on the game with his ability to read the game, and his quick decision-making.
His long punting gained hunks of territory.
Green Island was prepared to attack from all parts of the ground and extended its lead to 10-0 midway through the spell when hooker Frankie Elisara was up in support to score after a break that started at halfway.
Green Island led 10-0 at the break and then 17-7 after 13 minutes in the second spell when wing Mika Mafi scored after a sustained forward pressure that extended to six phases.
Winger Seb Hill scored both of Southern's tries in the second half.
The Magpies' revival started with experienced prop Kees Meeuws, who kept the ball tight in the forwards and made ground with the pick and go.
Southern dominated the set pieces, with its strong scrum and a dominant line-out.
But mistakes in the backline, pressure and strong tackling from Green Island spoiled Southern's attacking chances.
Flanker Josh Beer and centre Chris O'Neil were strong tacklers.
Green Island 17 (Mika Mafi, Frankie Elisara tries; Alex Davies 2 conversions, penalty goal), Southern 17 (Seb Hill 2 tries; Lewis Hancock 2 conversions, penalty goal).
Halftime: Green Island 10-0.
Referee: Ben O'Keeffe.
