Tasmania 264 (Villani 110, Stalenberg 75, Mushangwe 4-38) beat South Australia 241 (Webb 83, Coyte 4-30) by one run (DLS method)
South Australia required four runs from the final six deliveries with five wickets in hand under an adjusted target at Blundstone Arena on Saturday night.
Enter Coyte, who bowled Annie O'Neil (28 off 20) off ball one before getting South Australia skipper Jemma Barsby (28 from 17) stumped two balls later.
South Australia were still in the box seat, needing three runs from three deliveries when Coyte ran out Amanda-Jade Wellington knocking the ball onto the stumps in her follow-through.
Coyte, who finished with 4 for 30, then trapped Ella Wilson lbw on the second-to-last ball. South Australia needed three runs off the final delivery but could manage just one.
The dramatic last over capped a thrilling final in which both sides navigated rain delays and had their noses in front at different stages under DLS.
Tasmania become just the second team to secure back-to-back WNCL titles, leaving South Australia runners-up for the second year in a row.
The covers came on and off during South Australia's chase, which was headlined by an impressive 83 from youngster Courtney Webb.
Webb's departure in the 42nd over left South Australia at 5 for 185, before Barsby and O'Neil combined to drag their team back into the game.
South Australia were ahead on DLS when the covers went on again with six overs to go. But they were taken off again, leaving the final three-over scenario.
Tasmania had earlier let some runs go begging, with Villani's departure sparking a collapse of 7 for 20 as they were bowled out off exactly 50 overs.