The NCT is urging people to avoid a UK-based website that charges people fees to book appointments.
The site - nctservices.org site - resembles the official NCT website, and charges people a €76 fee for an NCT appointment.
However this money does not go to the NCT, and customers then have to pay the standard €55 for the test itself later - meaning the total cost is over €130.
The NCT has said the other website has "no affiliation" with them.
The website describes itself as "broker service that helps you get expedited NCT tests" - however the NCT says customers can get themselves on the NCTS priority list "quickly and easily" and for free on its own website.
Al Keating told The Pat Kenny Show his wife Googled 'NCT' on May 15th to book a test.
"This site came up so I took it as normal that it was an NCT site, went ahead and gave all her own details and car details.
"They asked for a payment of €76, so she gave the card details for two payments as normal.
"Then about 10 to 14 days later we got a notice that our car test was on the 28th of June.
"We went in to have our car tested on that morning, and they told us that we would have to pay €55.
"We told them we'd already paid €76, and they said that had nothing to do with them".
'The booking went through as normal'
Mr Keating said the site had made an actual booking at their NCT centre.
"Our confirmation of the booking came from NCT, and they had the details of it when we went in there on the 28th of June.
"The booking went through as normal".
He said the site looked very convincing, and he wants to warn other people.
"I really just want to make people aware - I'm not expecting in any way to be able recoup any of the €76 - but to make other people aware of the same service that's being offered".
Mr Keating said the car ultimately passed its NCT on June 28th.
'No affiliation with the NCT'
In a statement to Newstalk, the National Car Testing Service is warning people against using the other website.
"NCT wish to make it clear that this organisation has no affiliation with the National Car Testing Service in any capacity and urges the Irish public to proceed with caution and to only use the official NCT website," it said.
The NCT said it has posted warnings on its website and social media channels and has also reported it to the Competition and Consumer Protection Commission (CPCC), Google and the Gardaí.
"To date, we have had minimal referrals from customers who have been impacted from this website however, we are still urging the Irish public to proceed with caution when booking an NCT and to only use the official NCT website," the statement said.
"There is no need for anyone to pay a third-party website to book an NCTS or be placed on the NCTS priority list as this can be done easily at www.ncts.ie where a customer will only incur the cost of the test itself.
"NCTS would also like to warn drivers of the dangers of passing on their personal details, such as name, address, and credit card numbers to these sites. While such information is required to book an NCT, it should only be provided when booking a test through the official website," it added.
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