It was a spontaneous and off-colour joke, delivered to a crowd during a rendition of happy birthday, but the gag by musician Kyle Gass about the attempted assassination of Donald Trump has fallen flat.

The American comedy-rock group were performing in Sydney when Gass was asked to make that birthday wish.

Such has been the outrage that Gass and his more illustrious bandmate, actor and musician Jack Black, have cancelled the rest of "Tenacious D's" Australian concerts.

So when is a joke not a joke?