People have been warned to look out for "training money" that look similar to real banknotes.
A web user said she received a message from a friend saying a shop in Lok Wah Estate, Ngau Tau Kok, received a HK$500 banknote that looked genuine but bore a stamp saying "practice note."
She reminded people to be on the lookout for such banknotes.
"If you receive a stack of HK$500 banknotes, please check it one by one as someone received [training money] today in Lok Wah Estate," she said.
The size, color and pattern of the fake notes are the same as real banknotes, while the bank name, the amount, the serial number and the HSBC lion are clearly printed on the note.
However, there are two dotted lines at the right corner, and the words "Practice banknote. Sample. Only for practice. Circulation forbidden" have been printed in orange in the blank space on the right side of the note.
This type of banknote is designed for those who work in banks or financial institutions in the mainland to practice counting money.
However, the training money can easily be purchased on the internet at a low price.
A search on mainland shopping websites showed similar notes with amounts ranging from HK$10 to HK$1,000. They were being sold at around HK$10 per stack, with each stack containing 100 notes.
Some online sellers said these notes are color printed on both sides, while the paper texture, color and pattern are the same as real banknotes. Practice banknotes in other currencies like the yuan and the US dollar are also being sold on Taobao.
"The words are big, young people can realize it quickly, but it's easy for the elderly to fall into the trap," a web user said.
Another said people should inform police if they receive such notes.
A barrister warned keeping counterfeit banknotes could be against the law and added they should not be distributed.
On its website, the Hong Kong Monetary Authority said people need to ask it in writing if they want to reproduce Hong Kong banknotes.
Even if the HKMA allows people to reproduce notes, "the views of the relevant note-issuing bank should also be sought, in case the reproduction would in any way infringe its copyright."