Justin Bieber -- THREATENED BY AUSTRALIAN MAYOR ... Clean Up Your Graffiti, OR ELSE!!!

Justin Bieber picked the wrong city to spray tag, 'cause the mayor of Gold Coast, Australia just issued an ultimatum to the "pop princess" -- clean up your mess ... OR SING FOR ME!!!!!

Mayor Tom Tate just issued a video message to Bieber after the singer went graffiti crazy all over a hotel wall during a visit to the city last week.

"I've got a message for pop princess Justin Beaver," Tate said ... "We love your music but we hate your graffiti and your vandalism here in our city."

He continued ... "Mate, you've got two choices -- come back and clean up your mess or come back and sing at the [Mayor's Christmas Carols event] on the 7th of December."

It's unclear what will happen if Bieber ignores both requests ... but here's our advice for Mayor Tate: Don't hold your breath.