The atmosphere at “Book of Mormon” is starting to resemble the raucous streets of Times Square on New Year’s Eve, complete with booze and vomit, according to postings on a theater fan site.

“The house last night . . . was somewhat startling,” reads a post this week on Talkin’ Broadway from someone who saw the musical on Dec. 27. “A preponderance [of the audience] was inebriated, and became even more so, thanks to the sippy-cup revolution. Vomiting in the men’s room, and in one case, barely reaching it in time.”

The post further claims that the Broadway smash from Trey Parker and Matt Stone is attracting fratty fans of their “South Park” series who are not accustomed to attending the theater.

“They didn’t quite get what they were attending . . . [and hoped] that more alcohol would jump-start the fun,” wrote the poster, adding that an usher at the Eugene O’Neill Theatre confirmed, “drinking is prevalent” and gets “much worse on weekends and holidays.” “[The usher] said the current audience is typically white, male, under 40 . . . Hoping for ‘South Park’ in the flesh maybe, not quite getting it, and at times shocked that it’s a musical comedy.”

The audience member reports that, according to the usher: “Last week they had projectile vomiting down the stairs to the basement restrooms and resulting sanitation/odor issues (‘Think Mardi Gras in [New Orleans]’) . . . and someone who passed out in his seat, and couldn’t be roused by his drunken companion.”

However, a rep for the show denied the anecdotes and told Page Six, “We have never heard anything about any incidents occurring at any performances of ‘The Book of Mormon,’ and we have had no issues of any kind with our audiences inside the Eugene O’Neill Theatre.”

Another insider on the Broadway forum claimed to notice a similar trend of “frat-house behavior” at the now-closed “Rock of Ages,” with audience members arriving “blotto” and constantly getting up during musical numbers to relieve “beer-bruised bladders.” That poster proposed a solution: hawk “adult undergarments” at concession stands.