Terry Gornell put Dale in front five minutes into the second half of Saturday's League Two clash when he nodded home a right wing cross from Phil Edwards at the far post.
But that sparked the Shrimpers in to life and they equalised just seven minutes later when Assombalonga charged forward after collecting a pass from Marc Laird and fired home his 12th goal of the season from the edge of the penalty area.
Just a minute later, Paul Sturrock's side took the lead when a swerving left-wing cross from Michael Timlin caused Lillis all kinds of problems and, after the keeper could only parry it out, Tomlin was on hand to cooly slot home his fifth goal in three games.
Southend then made sure of the victory in the 79th minute when Assombalonga teed up Laird who chipped the ball over the advancing Lillis.
