Waikato have clinched a home semifinal in the NPC Premiership after a dramatic win over Otago in Rotorua tonight.

James Tucker's try 10 minutes from time ended a second half that produced four lead changes and a level of quality that belied the dreadful conditions.

Victory saw Waikato lock up second spot behind Hawke's Bay heading into next weekend's playoffs, while on this evidence Otago will provide a severe test for Manawatu when they travel north for their Championship semifinal.

What had been a sodden game in the first half hour tonight exploded as the players adjusted to the wet weather and the attacks began to shine amid the gloom.

Kicking for territory was the most popular early tactic but it was a cannier application of Josh Ioane's boot that brought the game's first try, with a grubber to the corner setting up Freedom Vahaakolo for a simple finish.

Waikato's defence was being sorely tested but the home side's fortunes flipped when Sam Fischli was sent off for making contact to James Thompson's head.

The Mooloos immediately set about making their numerical edge count in the most direct manner possible - packing down scrum after scrum and sending the seven-man Otago pack backwards.

That was the approach from a succession of penalties on the Otago line right on halftime, until hooker Rhys Marshall cleverly caught the defence napping with a quick tap to set up a try for Thompson.

Otago would have been relieved for the halftime break and the southern side's mood was further improved when, returned to their full complement, they regained the lead through Vahaakolo's acrobatic finish.

That advantage was short-lived as a brilliant break from Cortez Ratima sent Ryan Tongia over the line, but Otago were soon back in front after boldly turning down a shot at goal and being rewarded when Matt Faddes stepped his way over.

But with a third straight win in their sights, scrum time came back to haunt the visitors, conceding a free kick that allowed Tucker to crash across and decide the match.