"If the culprit is a local, I'm absolutely disgusted that someone would do this to visitors that bring money and entertainment to our town," Sgt Marshall said.
"Yesterday they were commenting on how hospitable the town has been; today someone has stolen something from them and taken from the fund raising they were going to provide back to our town.
"This is not something you're going to be able to display - anyone who sees it will know you stole it."
Australian Van Nationals organizer Matt Czerny was gutted when he discovered that the banner went missing in the early hours of Saturday morning and is offering a reward for its safe return.
"It's disappointing as this was one of the main items for our charity auction tonight," Matt said.
"All proceeds from the auction go to local Corowa organisations, so it would be great to get it back.
"We're offering a $500 cash reward for its return, and it can be anonymously dropped in to the local police station.
"I'm hoping we don't have to post footage from the CCTV at the main gate.
"Just do the right thing and bring it back."
Anyone who knows of the banner's whereabouts, or knows of anyone who might, is encouraged to do the right thing and either notify police or simply return it.
Contact Corowa Police on 6932 2899.
