Despite the fact that principal photography was completed earlier this year, a series of reshoots were ordered by producers in June and are due to commence this month.
Lost co-creator and Prometheus writer Damon Lindelof was brought in to rework the screenplay, focusing primarily on the film's third act, prior to the reshoots.
"It's just a big monster of a film, and it's got to work, and there's certain...how do I describe it? It's just gotta work a certain way," Pitt told HeyUGuys when asked about the nature of the reshoots, adding that such a decision is "not unusual".
He went on to say that he and his fellow producers realised that they "could do better" when they reached the editing stage.
"It's a monster of a film, and there's a lot of working parts to it," Pitt concluded.
In addition to producing, Pitt stars in the sci-fi drama alongside The Killing actress Mireille Enos, James Badge Dale, David Morse and Matthew Fox.
World War Z is directed by Quantum of Solace helmer Marc Forster, although it was reported earlier this year that Pitt was no longer speaking to Forster because he was so "frustrated" with the film's progression.
Additional reporting by Amie Parker-Williams
World War Z is set for release on June 21, 2013.