"We became aware of an app that was launched at the end of August asking for transport information for people in Auckland, Wellington and Christchurch. It claimed to put all your information into one place but it did require your AT HOP log-in and password."
The terms and conditions for AT HOP are that users do not share this information - in the same way they do not share credit or debit card information.
The spokesman said AT believed about 15,000 AT HOP cardholders gave their account details to this third party app.
AT HOP Group general manager Denise Verrall said: "We are talking about a robot which was sending multiple requests within milliseconds, whereas a normal user would only be able to click a refresh button every three or four seconds."
She said the security of the AT HOP system was never breached as the app did not access cardholders' financial transactions. It was only seeking their journey information.
Auckland Transport has met the backers of the app and they have modified their systems so they are not bombarding AT with requests.
Verrall said customers should never to provide account details to third parties.