The Reserve Bank is warning Canterbury businesses to look closely at the cash they are offered.
In the past fortnight, retailers there have accepted up to 50 fake banknotes of different values.
Typically, one or two counterfeit notes turn up in the banking system each month.
Reserve Bank spokesperson Peter Northcote said the notes would easily have failed proper checks.
It was important that people who handled cash knew how to test the look and feel of notes, he said.
"When you run your hand across the note, it's smooth and plastic-y; and when you look at the small bird printed on the note, it's got a coloured bar and if you tilt the note that bar will move up and down the note a bit like a hologram."
Tilting a note should make it look as though fine lines in the embossed feature move and reflect the denomination of the banknote in bright colours.
Knowingly passing a counterfeit note is a crime, and Mr Northcote said businesses should urge customers proffering fake notes to contact the police.