Constable Tom Taylor said the fake note turned up in the Rosebank shopping area on Monday.
It may have been a laser-printed copy of a $20 note.
Const Taylor said it was a very good reproduction and could easily fool most people.
However, the clear, see-through silver fern emblem found on a real bill is missing from the fake note, as is a thin metallic strip down the centre.
Its texture was also different.
To date, the fake note is believed to be the only one presented to a retailer, but Const Taylor urged all shop staff to be wary and ensure all banknotes handed over the counter were genuine.
"If someone was handing over a bunch of $20 notes with one of these [fake] ones in there, it would be pretty hard for anyone to spot the fake," he said.
Const Taylor said fake notes should be taken to the Balclutha Police Station.
