Wednesday, now winless in six matches, needed only five minutes to go in front when Jermaine Johnson fired into the far corner. But Posh hauled themselves level 11 minutes later as top-scorer Craig Mackail-Smith snapped up a rebound after a Lee Tomlin blockbuster had been parried.
Parity was short-lived as Gary Madine pounced for his second goal in three Wednesday appearances in the 21st minute but again the hosts quickly hit back as skipper George Boyd drilled a 20-yard drive through the legs of Nicky Weaver.
Chris Sedgwick put Wednesday back on top after swapping passes with Neil Mellor 10 minutes before the break - and that was the way it stayed until the turning point came when Darren Potter saw red 20 minutes from time for hacking down Mackail-Smith.
Posh wasted little time in levelling as Boyd bulldozed his way through before rifling past Weaver from close range. And the hosts hit the decisive blow with six minutes to go as substitute Nathaniel Mendez-Laing weaved his way through the Wednesday defence to convert a brilliant Tomlin pass at the second attempt.
And Posh added salt into Wednesday wounds as Mackail-Smith tucked away a stoppage-time fifth into an empty net after Owls keeper Weaver had gone up for a corner at the other end.