The 21-year-old, who plays Gregory Goyle in the wizard franchise, made the admission during a Harry Potter symposium prior to his recent drug arrest, according to MuggleNet.
Herdman was part of a panel comprised of several other actors from the movie series, while he admitted during a Q&A session that the two-part final film would be split after Harry, Hermione and Ron are captured by the Snatchers.
However, it is thought that director David Yates may change the decision once shooting is completed.
Herdman is also reported to have claimed that his character will die in the feature, instead of Jamie Waylett's Crabbe, after the star was arrested on cannabis charges and is not expected to return for Deathly Hallows.
Actress Emma Watson recently said that filming on the instalment has differed from that of any of the previous movies.