Ellisa Cox break early in the first half engineered a score from Chantelle Savage, which gave Southwold a 7-0 lead.
But sensing another romp, Peterborough relaxed and their complacency allowed the heavyweight Southwold pack to take control of the scrum and line out.
A contentious decision disallowed three points for Peterborough and Southwold capitalised by pouncing on Peterborough mistakes to score a second try.
Just before half time Peterborough luck turned scored a try - 7-12
Second half Peterborough now found their stride and the scrum began to dominate the flagging home pack.
Under pressure, Southwold’s frantic defence was penalised for repeated infringements and eventually suffered a yellow card for continued ill discipline.
Peterborough winger took the game to the Southwold 22, to squeeze through the Southwold defence. Final score 12-12