The hosts had the game won by the half-hour mark as a blistering start blew Southend away and gave them a 3-0 lead, which became 4-0 after the break.
Lee Tomlin got the ball rolling when he stabbed home George Boyd's pass after 12 minutes and their advantage was doubled 16 minutes later as Paul Taylor beat two players with a surging run before he slotted in.
And the home fans did not have chance to stop celebrating before they had another goal to cheer as straight after the restart and it was one of real quality.
Joe Newell picked the ball up and ran with it before delightfully chipping over hapless Southend goalkeeper Cameron Belford with an effort that went in off the underside of the bar.
The second half was all about restoring some pride for Southend and they almost got a goal back shortly after the restart but Kane Ferdinand's looping header hit the crossbar.
But their night was to get even worse when Posh made it four just before the hour when Tomlin played a delightful reverse pass for Boyd to fire in.
The visitors' Ryan Hall rattled the woodwork again soon after, but it was a night to forget for the Shrimpers.