Chasing 144 for victory, two-time defending champions Brisbane were cruising at 4-119 before they fell apart in a bizarre climax to the semi-final on Thursday night.

The collapse cost them a shot at the competition's first ever three-peat, but will go down as one of the Thunder's greatest wins in their franchise's history - male or female.

Teenager Hannah Darlington was the hero, taking 3-19, including two wickets with yorkers in as many balls in the 18th over before the game was wrapped up in the next.

"The whole game we believed," 18-year-old Darlington said.

"We knew as soon as we got into the lower order we could pull it off.

"That (yorker) is a nice strength to have I do go to it a lot and it's a wicket-taking delivery.

"It's something I have practised a lot. And I have done for a couple of years now."

Laura Kimmince had earlier looked like she had set up the win for the Heat with 37 from just 17 balls but she threw her wicket away at a crucial point.

After both Georgia Vole and Amelia Kerr were run out in the same over with the score on 119, Kimmince appeared as if she would get Brisbane home with her hard hitting.

Batting fearlessly throughout the tournament with a strike-rate above 200, Kimmince whacked four sixes down the ground.

She had been lucky to survive her first ball from Sam Bates, which bounced off her body and into the stumps only for the bails to stay on.

But the match swung again when Kimmince tried to reverse slog Bates with the Heat needing 15 runs from 19 balls and was bowled off her pads.

Darlington claimed two wickets the next over but missed out on a hat-trick.

Unheralded South African Nadine de Klerk also hit a run-a-ball 27 for Brisbane, before she was run out taking on the arm of Tammy Beaumont.

"It's very disappointing," captain Jess Jonassen said.

"It probably just came down to the fact that we didn't really have many firm plans in terms of how to take on Darlington."

Captain Rachael Haynes had earlier been the calm head for the Thunder, hitting an unbeaten 48 from 44 balls.

The left-hander was measured throughout, using her feet well to the spinners, before helping her side take what proved a crucial 15 runs from Delissa Kimmince's final over.

The win puts the Thunder in to their first final since their victory in the tournament's inaugural season, in 2015-16.

Australian Associated Press