Down 31-37 with time up, Waikato hammered away at Auckland's line and the pressure told as replacement forward Hikairo Forbes barrelled over the line and first five-eighth Trent Renata, who had been in patchy form with the boot, nailed the conversion from 15m inside the righthand touchline to send Waikato through to next weekend's final against Canterbury at Christchurch.
Waikato started slowly but wound into gear either side of halftime, scoring four tries in the space of 24 minutes to lead 28-20 with 20min remaining.
Auckland stayed calm under pressure and looked to have sealed the match when they led 37-28, but Renata kicked a penalty and Forbes' try earned them a stunning win -- their sixth in succession -- and a first final appearance since 2006, when they won the title.
"I can't believe it, I'm not a religious man but I think I might go to church tomorrow to say a few prayers," Waikato captain Nathan White told Sky Sport.
"We knew we had to stick it out and we did in the end. We knew we needed to get the ball and to be one point up at the break without much possession, we were pretty happy.
"In the second half we got a little bit more ball and we managed to hold on. The way we finished at the end, I can't believe it."
Auckland were more composed than Waikato in the first 30min but they could not turn their advantage into a healthy lead.
They started well enough with captain Benson Stanley capitalising on a Waikato turnover deep inside their own half before brushing off some big forwards in the backline to score the opening try after Renata had kicked an early penalty.
Renata and his opposite, Gareth Anscombe, traded penalties but as Auckland's tight finals-football approach looked to be getting the better of Waikato's looser style, the visitors clicked into gear to score two tries in 10min leading into halftime.
Fullback Sosene Anesi was on hand to clean up the scraps and dot down in the corner after explosive centre Save Tokula had beaten Ben Atiga on his outside to provide the initial thrust.
Auckland reclaimed the lead after 35min when halfback Toby Morland darted down an extremely short blindside and found Liaki Moli in support, the big lock returning the ball to Morland who scooted over the line.
But Waikato took an 18-17 halftime lead when they returned to what they know best, blindside flanker Dominiko Waqaniburotu scoring the try after a 10m lineout drive.
Lock Romana Graham extended Waikato's lead early in the second half when he was on the end of a sweeping move from inside their own half.
Auckland stayed in touch courtesy of Anscombe's boot but Waikato scored their fourth try through right wing Henry Speight, who collected a neat flick-on pass from Graham to score in the corner and stretch the lead to 28-20.
Auckland coach Mark Anscombe, sensing his side was in trouble, turned to the bench with immediate results.
Replacement halfback Auvasa Faleali'i scored with his first touch of the ball when he spied a gap around the ruck 10m out from Waikato's line, and a third Anscombe penalty put Auckland ahead 30-28.
Auckland fullback Brent Ward gathered in a charged down kick and sent left wing Chay Raui on a straight run to the line to make it 37-28, but the last 10min were all Waikato's.
