The 30-year-old former Hibernian goalkeeper brought down Craig Mackail-Smith and Posh never looked back once Aaron McLean stepped forward to sweep the spot-kick into the roof of the net.
Bees boss Terry Butcher had to sacrifice midfielder Charlie Ide to bring on substitute goalkeeper Clark Masters, whose first task was to pick the ball out of the back of the net when sent wrong way by the ice-cool McLean.
The Posh leading scorer went on to rattle up a hat-trick before the break and between well-taken 14th and 41st minute shots that brought his season's tally to 15, McLean amazingly missed a second-penalty firing against the woodwork when Glenn Poole was adjudged to have handled.
To their credit, ten-man Brentford persevered with Lee Thorpe and Emile Sinclair up front, but they hardly saw anything of the ball and Posh powered on with Chris Whelpdale scrambling a fourth from close range in first-half added time before George Boyd made it five just four minutes into the second period.
Already assured of his biggest win since arriving at London Road back in January, Posh boss Darren Ferguson could afford the luxury of taking McLean off on the hour but the goals continued to come.
Mackail-Smith used his pace to get on the scoresheet after 56 minutes and the statistics became even more depressing for Brentford when substitute Rene Howe hit his first for the club and Posh's seventh 20 minutes later.