Tasmania won the summit clash by one run under the Duckworth-Lewis method.
South Australia required four runs from the final six deliveries with five wickets in hand under a revised target in the 50-over day/night match at Blundstone Arena on Saturday night.
But two run outs and three wickets — all by pacer Sarah Coyte — in the last over turned the match upside down. Coyte was also involved in one of the two run-outs.
In the dramatic final over, Coyte bowled Annie O’Neil (28 off 20) off the first ball before getting South Australia skipper Jemma Barsby (28 from 17) stumped two balls later.
South Australia were still in the hot seat, needing three runs from three deliveries when Coyte ran out Amanda-Jade Wellington (1) by knocking the ball onto the stumps in her follow-through.
Coyte, who finished with 4 for 30, then trapped Ella Wilson (0) lbw with the penultimate ball. South Australia needed three runs off the final delivery but could manage just one.
Tasmania become only the second team to secure back-to-back WNCL titles, leaving South Australia runners-up for the second year in a row.
Captain Elyse Villani had earlier spearheaded Tasmania to 264 all out from their 50 overs with 110 from 126 balls, her third century in the last four innings.
The covers came on and off during South Australia’s chase, which was headlined by an impressive 83 from youngster Courtney Webb.
South Australia were 220 for five before rain stopped play with six overs to go. They were then five runs ahead of the DLS target.
On resumption, South Australia’s target was revised to 243 in 47 overs as the game lost three overs. They were 239 for five before the last over.