The counterfeit money being used in Katherine was likely bought off the internet.

The fake $50 and $20 Australian notes are sold as “training aids” and manufactured in China.
On the internet today, you can buy five of the fake Australian banks notes for a real 10 bucks.
For the $10, you get a fake $5, $10, $20, $50 and $100 notes.
While some ads say they are novelty items, others say they are bank training aids.
The fake notes clearly have Chinese writing on them and none of the many security aids the “real” money has.

But police say the crooks prey on busy shop staff who might not have the time to check the fake money too closely.
Northern Territory Police seized an amount of this counterfeit money in Katherine last weekend.
Detective Senior Sergeant Mark Malogorski said police urge Katherine residents to be aware of counterfeit money being distributed in Katherine over the past week.
“We ask the public to please be vigilant of fake notes being circulated,” he said.
Police are asking anyone who has received counterfeit money in Katherine and surrounding areas to notify them. Anyone with information is urged to contact police on 131 444 or Crime Stoppers on 1800 333 000.