A 'technical error' caused problems for motorists using Sand Street car park.
The government says a problem with the provider who processes debit and credit card payments.
No other car parks are affected and it doesn't affect drivers using UniTickets and ParkSmart cards.
Anyone who wants to pay via that method can pay in person at the main Parking Control office. Opening hours have been extended to 08:30-17:30 to meet demand.
Drivers can also pay over the phone by calling the Administration Office on 448660. If the line is busy, drivers can leave a message requesting a call back so they can make their payment.
“We apologise for the inconvenience this has caused for drivers. We are working closely with our merchant provider to restore the service; we hope to resolve the issue this week. In the meantime, we have doubled the number of staff working on-site at Sand Street, and have two staff taking payment via phone.
"While we understand that this may make it more time-consuming to make a payment, we urge drivers to work with our staff to make their payment in full. Drivers can also purchase 5 or 10 unit UniTickets with their shopping to make their payment more easily." - Paul Gibaut, Parking Control Manager.