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."
Charleville resident Laurie Parker told Australia's ABC: "There's other things we've been concentrating on here and we've been wanting to get rain [to prevent drought] and pick up the economy and things like that, but certainly those sort of birthdays are important to celebrate."
The town of about 3,300 people will make alternative plans.
"We're still planning on having some sort of celebration next year - the event staff are currently trying to organise what form the event will take at this stage," Mayor Liston said.