The Black Caps and Otago left-armer is a different bowler with a red ball in his hand and took five for 45 to help dismiss Central Districts for 188.
Former international spinner Mark Craig also accelerated from the doldrums with four for 43.
Their effort helped the Volts take a handy lead of 101 into their second innings.
Otago lost some momentum with the early departure of openers Hamish Rutherford (10) and Brad Wilson (9), while Anaru Kitchen completed a terrible match with the bat when he dragged a delivery on for two.
At stumps, Otago was 69 for four, which took the gloss off Wagner’s 30th first-class five-wicket haul. But it was a tremendous effort by the 31-year-old.
"This is where Wags really comes into his own," Volts coach Rob Walter said.
"To be fair, the whole unit bowled well but obviously Wags was the guy who cashed in with wickets along with Mark Craig.
"He bowled with great energy. He bowled long spells and I think his dismissal of Tom Bruce was outstanding.
"He put him under a lot of pressure and bowled really smartly to eventually get him out caught."
Wagner really laboured during the limited-over campaigns. By his own admission, he tried too hard and was too focused on cracking into the Black Caps one-day and twenty20 sides.
He was trapped either bowling too short or too full and got clobbered. He took just two wickets in the one-dayer at a Bradmanesque average of 97.
But with the extra protection afforded to bowlers in the longer format, Wagner was back doing what he does best.
He used the short ball to account for opener Ben Smith late on day one, and wasted no time removing the nightwatchman Ajaz Patel early yesterday morning.
Brad Schmulian got an accurate bouncer and fended it into the slips, while Greg Hay was surprised by a fuller delivery and got a leading edge back to a delighted bowler.
Bruce was worked over and eventually edged a good-length ball to complete the five-for.
Wagner’s return to form was a timely reminder of just how good he is on form, in the lead up to the test series against England coming up later this month.
Craig was outstanding as well. He ran through the tail and picked up the key wicket of the dangerous Jesse Ryder.
Central Districts captain Will Young top-scored with 49. His dogged effort was cut short by a sharp rising delivery from Michael Rae.
Otago’s top-order batting frailties have left the match evenly poised with two days to play, though.
"Maybe we have our noses slightly in front but not by a hell of a lot."
The first hour will be crucial to establishing a good lead. The Volts still have plenty of batting to come and Walter is hoping his side can build its 170 lead to 300 or so.
