Champion Caversham stays top with nine points and a goal difference of 12 after three rounds. Lurking in second place, Green Island has a goal difference of seven.
Mosgiel's defenders must have been kicking themselves when Green Island opened the scoring after 26 minutes. As the Plainsmen resembled Morris dancers, organising their defence at a free kick, young Stevens stayed alert and calmly stroked in his goal.
Not having asked for 10m, the attacking side can take the kick without any referee's whistle, and Stevens did just that, to Mosgiel's huge embarrassment.
The match had been evenly balanced, with Green Island patiently stroking passes around and then looking to harness the pace of Stevens, while Mosgiel won good possession and unleashed Tim Dunn and Regan Coldicott.
The latter boosted Mosgiel morale with a splendid shot on the run that whistled into the Island net just before halftime.
However, Coldicott and Aaron Corkhill left the field injured. Green Island took the lead through Stevens then clinched the win with a Nathan Gunn penalty. Phil Kelly was Mosgiel's other scorer.
At Ellis Park, Roslyn-Wakari was hard-pressed to dominate, despite taking an early lead when Sam Mepham beat Grants Braes keeper Luka Velebit, who went on to have a fine match.
Despite Roslyn's superior ball possession, Braes dug in to keep the score 1-0, and it was late in the match before Fraser Cameron nodded in a close-range header to bring relief to Roslyn coach Colin Thom.
Braes was well led by centreback Joh Mpondo, and some clever play by Alfred Kydanh, Alijan Rajabai and Patrick Ebanda threatened Roslyn, which missed Brazilian Guilherme Melo after he went off with an ankle injury.
The safety goal arrived with some style, as Cam Attwell slapped in Roslyn's third goal after some first-time passing but, as an indication of Braes' unstinting effort, big centreback Mpondo rose at the far post to bullet in a fine header in the 89th minute to make the final score 3-1.
In Invercargill, Northern continued its goal-scoring form and beat Old Boys 3-1, with goals from player of the day Angus Milne (two) and a free kick by Norbert Dreyer. Old Boys' Andrew Cunning scored in his last game for the Invercargill side.
At the Caledonian Ground, University's Ifeanyi Unamagu and Henry Flood scored for the students, but Dunedin Technical did not give up and drew level with goals from David Hayman and Josh Stewart for a 2-2 draw that pleased neither coach.
As usual, it is Caversham setting the pace, and that Ridden hat trick, plus a brace from Ant Hancock and goals from Wil Smith and Evan Jones, are a clarion warning to the other FPL teams.
