Posh were given a footballing lesson with Swansea enjoying long spells of possession, although despite the increasingly animated urgings of manager Paulo Sousa from the touchline, they had nothing to show for it by the break.
Good defensive work by Scott Griffiths, Craig Morgan and Exodus Geohaghon kept Posh in touch, before Andrea Orlandi saw a shot deflected for a corner he took himself and from which Gorka Pintado's drive also went wide.
Posh hardly figured as an attacking force and in the opening half and achieved nothing more than a solitary corner.
It was 51 minutes before they managed to get even remotely close to the target when Lee Frecklington drove a Chris Whelpdale cross wide.
Four minutes later there was an unlikely Posh breakthrough as Wales international defender Morgan flicked on a towering throw from recent Kettering capture Geohaghon for Whelpdale to volley in from the edge of the area with the ball going past Dorus de Vries off the inside of a post.
Whelpdale was inches away from a second when launching himself at George Boyd's low cross and the one-time Billericay winger then forced de Vries into a smart save.
Veteran front man Trundle, on loan from Bristol City, entered the fray as a substitute after 70 minutes and put Swansea into the driving seat after showing coolness to conjure up an 84th minute leveller.
He went one better a minute into added time after turning Geohaghon before beating home keeper Joe Lewis.
But to the relief of new Posh boss Mark Cooper, there was still time for McLean to force in his seventh goal of the season.