Heavy downpour resulted in 10-overs-a-side affair as Northern skipper Daniel Flynn opted to field.
Kusal smashed his way to 37 off 20 balls with seven boundaries as Gunathilaka contributed 39 off 26 balls with two fours and three sixes as their opening stand of 55 laid the foundation for a challenging score.
However the left-handed duo were helped by the profligacy of the Northern fielders, who dropped as many as six catches, three each of both openers.
He was then dropped on 33 when he tried to pull Scott Styris and skipper BJ Watling standing up failed to gather the reflex catch.
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On the same score off Styris' next over, he was dropped as Trent Boult wasn't standing at the deep mid-wicket boundary line and the result was a catch that went through his fingers for a boundary. However Styris had the last laugh when Kusal tried to whip another one off his legs but Boult standing at square leg held onto the catch.
Desperate to break shackles, Gunathilaka lofted Styris over cow corner which the fielder failed to latch onto as ball went over boundary ropes. However Gunathilaka's next two sixes off Anton Devicich's left-arm spin were clean hits as he made up for the initial waste of deliveries.
Once he was played on by Boult, Southern Express's innings lost a bit of momentum.