The home side started brightly, but Northern stuck to an effective counter-attacking plan, springing forward from a well-marshalled defence, and deservedly won the match with a couple of exceptional goals.
Will Dickinson ran on to a perfect through pass in the 14th minute, slicing through the Roslyn central defence, and calmly shooting past Peter Evans in the Roslyn goal.
Roslyn replied with a spell of pressure that earned an equaliser through Jackson Rodeka five minutes later, but the home side indulged in too many build-up passes, and attacks foundered against the resolute Northern defence.
Replacement keeper Nick Grant was well protected by strong matches from Cam McPhail, Norbert Dreyer, and the Wilkinson brothers, Craig and Arran.
Northern regained the lead after 35 minutes, when a weak clearance was powerfully laced goalwards by Tom Maclean, and the ball hit Roslyn's net before heads could turn.
Young Aajay Cunningham and Sam Mepham generated hope for the locals, but too often passes went astray, and the extra pass was opted for instead of shooting at goal.
Tim Smith clinched the match for Northern when a free kick rebounded his way, and again Roslyn keeper Evans had to fish the ball from his net.
The win moves Northern into fifth place, three points behind Roslyn, but the unbeaten Dunedin Technical still dominates the FPL with a five-point lead, averaging almost six goals per match.
This week, Technical beat Grants Braes 4-0, and striker Aaron Burgess grabbed another hat trick to stretch his lead as top league scorer to 13 goals, with a third of this season's fixtures played.
Caversham returned to form with an 8-0 hammering of University at the Caledonian Ground, and player-coach Richard Murray leading the way by scoring himself.
Mosgiel earned a valuable 3-1 away win in Queenstown with Jordan Swaney and Steve Cromb getting among the goals, while Matt Small scored for the Rovers.
Bottom side Green Island scored through Cody Robinson in Invercargill, but the home side, Spirit FC, hit three goals through Kieran Van Loon and Andrew Ridden (two).
