The ASB bank has been affected by card fraud and is contacting thousands of customers by phone and mail to warn them that their details may have been disclosed.

The letter states that, “ASB was recently made aware of a situation that might have caused some card details to be disclosed.”

Those receiving the letter have done so because their details are thought to have been disclosed.

Customers are also receiving phone calls saying that "thousands" have been affected by the fraud.

“As the matter is currently under investigation, we are unable to provide any further details, but I assure you that the matter is being dealt with,” the letter continues, from ASB’s Head of Cards, Stephen Powell.

“Please also be assured that, as you have not contributed to this matter, you will not be responsible for any related fraudulent transactions on your account.”