Southern led 2-1 at halftime following goals from Hayden Shaw and Adam Clifford.
Second half goals from Chris Ashton, Jared Jones and Kane Russell sealed the victory.
For Northland, Duncan Pearce, Josh Miller and Sean Laidlaw got on the score sheet.
The win has cemented Southern's spot in the top four but with two round-robin matches remaining the Dunedin-based team could slip out of contention.
Southern plays an in-form Midlands unit tomorrow and Auckland on Thursday.
A win in either of those fixtures could be enough to clinch a semifinal berth.
A win in both games could see Southern claim pole position.
That is a scenario which has previously been unthinkable.
The combined Otago-Southland is used to being closer to the bottom of the competition table and, in six years, its best finish has been fifth.
But with the Black Sticks unavailable this season, and most of the National Hockey League sides experiencing significant turnover in personnel, the competition is wide open.
That has paved the way for Southern to sneak into the top four and the team is desperate to take its opportunity.
Midlands had a 5-3 win against Capital yesterday and shape as a tough opponent, and Auckland was unbeaten after its opening four matches.
Southern coach Dave Ross was upbeat but also realistic about his side's semifinal prospects.
"Just the way that Midlands play I think we have an opportunity," he said.
"But I think we'd have to be at our best and they'd probably have to be slightly under.
"We'll be giving it a good nudge but they played exceptionally well [yesterday].
"No-one had smacked in five goals against Capital so they are on a bit of a high.
"But Auckland, in all honesty, will be difficult. They have been playing really well and beating everyone comfortably."
The Southern Storm, though, is out of the running.
The women's team was beaten 3-1 by North Harbour and is anchored at the bottom of the competition table with two round-robin matches remainingPhillipa Symes managed one goal for the Storm but a brilliant hat trick from Petrea Webster sealed its fate.
The Storm plays Northland tomorrow.
