Tim Dalman delivered a right-wing cross for Sam Mepham to twist round a defender and slam in Roslyn's first goal after 40 minutes.
On a playing surface hardly conducive to the short passing game, both sides pushed forward in attack, but while Northern suffered from repeated breakdowns, Roslyn produced convincing passing movements that peppered Northern's goalmouth.
"That was our best performance so far and, with steadier finishing in front of goal, we could have doubled the score," Roslyn coach James Vaughan said.
A clever looping backheader by Peter Rae sent a cross over stranded Northern keeper Tom Galloway and, with James Watson in top form spraying passes, Roslyn's Fraser Cameron, Tim Mather and Tom Connor made penetrating runs forward.
Steady games by Northern defenders Cam McPhail and Arran Wilkinson blunted some attacks, and Tim Smith occasionally showed pace in attack.
A late red card to Sean Brand hardly helped Northern's cause.
At the Caledonian Ground, Green Island competed well against Dunedin Technical in the first half before Aaron Burgess hit his fourth goal of the season, and in the second spell an Island own goal and an Alistair Rickerby goal finalised the 3-0 result.
Initially, some hard running by the Island pressured errors in the Technical defence, as Cody Robinson and Jordan Cudd-Anderson chased half chances, but Technical settled as fullbacks John Dale and Tristan Prattley pushed forward, Justin Flaws delivered from wide positions and Rickerby drove forward round the lurking Burgess.
At Memorial Park, Mosgiel matched Caversham until a 37th minute goal by Patrick Fleming.
Then the Plainsmen showed resilience as Morgan Day used his height to head an equalising corner only four minutes later.
The save of the match was a great effort from Caversham keeper James Waggett when he stretched to prevent what could have been a critical own goal.
Fleming re-established Caversham's lead in the 74th minute, and Mike Abbott conceded a penalty that Robbie Deeley drove in for his 10th goal of the season to clinch the 3-1 win.
In the neck-and-neck race at the top of the FPL, Caversham's goal difference advantage dropped by one, making its superiority over second-placed Technical now only four goals.
At Ocean Grove, a robust challenge in the air by University's Dave Stewart upset Grants Braes keeper Josh Columbus to net the only goal of a fiercely contested game.
Queenstown moved up to fourth-equal with Mosgiel by beating the Spirit in Invercargill, where the goals came from Sam Jasper and Nick Bishop.
