COUNTERFEIT $100 banknotes are believed to be in circulation in The Hills, and the command's crime manager Detective Inspector Gary Bailey says they are "a remarkably good photocopy on quality paper".
He said the notes can be torn, as distinct from original $100 notes which are made from polymer (plastic).
"Be on the look-out," he said.
Detective Inspector Bailey said counterfeit $100 banknotes were used to make purchases at a juice store at Rouse Hill Town Centre at 7pm on Thursday and a clothing store five minutes later.
Fake notes were also used at Castle Towers on Friday at about 1.30pm.
Police want to speak to two men, of Middle Eastern appearance, who they believe may be able to assist them with inquiries.
One man is 185cm tall, of slim build, with poorly spaced teeth. The other man is 170cm, stocky, with black hair, shaved sides and a rat's tail.
Suspicious banknotes are sent to the Reserve Bank of Australia which is responsible for the issue, reissue and cancellation of Australian banknotes.
According to the RBA, under ultraviolet light serial numbers on the back of a $100 banknote glow and a patch showing the value of the banknote becomes visible.
Intaglio (raised dark printing with a distinct feel) is used for the portraits , and multi-coloured and multi-directional patterns of fine lines appear on each side of the note.
The words ‘ONE HUNDRED DOLLARS’ and the number ‘100’ are microprinted.
When the banknote is held up to the light an image of the Australian Coat of Arms can be seen faintly, under other printing, and a seven-pointed star within a circle is formed by four points on one side of the note.
On all genuine banknotes the window should be very clear.
Inside the clear window on the $100 banknote is an image of a lyrebird, along with embossing of the number ‘100’.
TEST YOUR KNOWLEDGE: There are seven security feature elements to find on each banknote. Can you spot them all? Click here to play EYE SPY and test your knowledge.
■ Date of first issue: May 15, 1996
■ Size: 65mm by 158mm
■ Designer: Bruce Stewart
TIP: To determine if a suspect banknote is a counterfeit, compare it with a banknote known to be genuine.