The town of Charleville, in Queensland, had begun planning festivities for next year to mark what locals believed was its official recognition in 1868.
However, a check of state records showed the historic moment had actually taken place in 1865.
Mayor Annie Liston said it was a disappointment.
"We put so much budget towards it, then we found out it was gone. It was the biggest shock to all of us, I can tell you," she told the BBC.
"We were all prepared to have this big shindig next year and [the date] is not even correct."
