It was Sheffield United's first victory of the Championship season but won't have made good viewing for Posh fans - despite the two teams going into the break with one goal apiece.
The Blades started strong with Ben Osborn surging down the left flank and firing a cross into Iliman Ndiaye, but Posh keeper Christy Pym palmed the ball away from danger.
But he wasn't able to keep Ndiaye off the scoresheet for long and Sheffield United were one up within 15 minutes.
Sheffield continued to dominate after their first goal, creating a number of opportunities, but conversion was weak and Jack Marriott was able to bring the visitors back to level terms in the 39th minute. He found himself clean through on goal and slotted the ball neatly into the bottom-left corner.
The Blades came out into the second half determined to translate possession into points and it took them just six minutes to reclaim the lead with some nifty teamwork from Billy Sharp and John Fleck.
And just two minutes later an Osborn shot from close range secured the lead even further.
The fourth goal for Sheffield came in minute 68 in a stunning finish from Morgan Gibbs-White and Ndiaye took number five only two minutes later.
Posh continued to fight, with substitute Jonson Clarke-Harris getting a goal back from the penalty spot to make it 5-2 in the 78th minute, but the home side had the final word when Osborn made it six for Sheffield in stoppage time.