Supermarket chains and customers are on high alert as a new scam involving the popular Prezzy Card leaves some out of pocket.

Particularly in the Waikato's Matamata-Piako District, people had bought a Prezzy Card only to find the packet had a bogus card inside.

Police have had reports and are investigating, and one Matamata shopper warned others of the concerning scam on social media.

'Just a heads-up. If you purchase Prezzy Cards from New World or Countdown etc there is a scam going around where the Prezzy Card is swapped out for a random card,” the Matamata shopper says.

'Mine was a deleted Farmers club card.

'The shop activates the card and the scammer gets to use the Prezzy Card, and you get left with nothing. Please check the cards as you purchase at the till to save the hassle of the refunding and the shop being out of pocket.”

Both New World Matamata and Countdown Matamata confirm they are dealing with the scam at their branches.

Cards are immediately blocked when incidents occurred.

'We have seen a few instances of this scam and in each case we've been able to block the original Prezzy Card from being used fraudulently and have loaded a new card for the customer,” a Countdown spokesperson says.

'Our team are well-trained in spotting these types of scams, and we've let all of our stores know to watch out for this particular one and how to handle the situation if they see it happening.”

New World Matamata declined to give further comment about the matter and directed Stuff to their national office, which is yet to respond to Stuff's request.

In a statement, police say they have been made aware of the scam and are investigating.

It's a relatively new issue, police say, and they are not willing to go into detail about how exactly the scam is carried out or advice for retailers due to possible copycat behaviours.

'Waikato Police are aware of 12 reports in Rotorua and the rest are in Waikato,” Waikato Police says.

'Police are following strong lines of enquiry to locate those responsible and will be providing prevention advice to impacted retailers.

'Anyone who has been the victim of a scam of this nature is asked to contact Police on 105.”

-Stuff/ Te Aorewa Rolleston.