The Posh striker reacted sharply after Mark Phillips could only head down a left wing cross from Michael Bostwick and, after composing himself, he sent a composed poked finish in to the bottom right hand corner of the net from eight yards.
Peterborough then came close to doubling their lead in the 19th minute when Paul Taylor went clean through on goal but the striker’s low shot was well blocked by the legs of Cameron Belford.
However, Taylor did not have to wait long to get on the score-sheet as he doubled Posh’s advantage just nine minutes later.
The 24-year-old latched on to a weak header from Chris Barker half way inside the Blues half before charging towards goal, evading two challenges and sliding the ball past Belford from just inside the penalty area.
Within a minute, things got even worse for Southend as Peterborough made it 3-0 with Joe Newell sending a fantastic 20 yard chip in to the top right hand corner of the goal via the underside of the crossbar.
A shell shocked Blues side was then forced to in to making their first change of the night in the 39th minute when Neil Harris limped off to be replaced by Freddy Eastwood.
And, with their new look strike-force, Southend almost netted five minutes in to the second half when Kane Ferdinand sent a looping header from Anthony Straker’s left wing cross against the bar with Eastwood nodding the rebound over from eight yards while under pressure.
With Blues beginning to look a bit brighter, Ferdinand then headed a right wing cross from Phillips wide of the right post but Peterborough replied to extend their lead even further in the 58th minute.
This time Tomlin was the provider as he cut open the Southend defence with a fine through ball which enabled George Boyd to fire home, right footed, from eight yards.
Paul Strurrock’s side then twice came close to getting on the score-sheet with Ryan Hall sending a powerful 20 yard shot fizzing just past the right post before he also sent a free-kick from a similar distance crashing against the crossbar.
After that, Blues soon made their second change of the night with Kevan Hurst replacing before Seedy Njie came on for Elliot Benyon up front in the 76th minute. Njie almost worked his way on to the score-sheet when he volleyed a Phillips header towards goal but Posh keeper Bobby Olejnik was able to keep out effort.
And Southend will now be hoping for far more when their start their League Two season at home to Accrington Stanley on Saturday.
Southend United: Belford, Clohessy, Phillips, Barker, Straker (Hurst 67), Hall, Ferdinand, Leonard, Straker, Harrris (Eastwood 39), Benyon (Njie 76).
Subs not used: Bentley, Coughlan & Woodyard
Peterborough United: Olejnik, Alcock (Little 61), Brisley, Zakuani, Ntlhe, Newell (Swanson 68), Frecklington (McCann 76), Bostwick, Taylor, Tomlin, Boyd
Subs not used: Day, Knight- Percival, Ajose & Clarke-Harris