It is believed the attack is isolated.
The agency said it takes cyber security “extremely seriously.
“We have activated our security protocols and are working with our expert partners to resolve the issue as quickly as possible, however, we anticipate it may take until early next week to fully restore these services,” AT said.
“We will fully investigate the incident and provide further details following the completion of that investigation.”
The Hop card system was designed, developed, and implemented by the French multinational Thales Group.
The cyber attack came ahead of a planned upgrade.
In June, AT said Auckland commuters will be able to pay for public transport with a simple swipe of a bank card or smartphone within the next year, using a debit card, credit card, Apple Pay or Google Pay.
Brett Callow, a threat analyst with New Zealand-based firm Emsisoft, said there were no immediate indications of which ransomware gang was behind the attack.
Callow monitors the dark web for customer or company data put on sale following ransomware attacks.
“James Babbage from the UK’s National Crime Agency recently said, ‘Ransomware is fundamentally about criminal monetisation of cyber vulnerabilities. The most effective systemic response is preventing future attacks by investing in increased resilience and better-protected systems.’ His comment is spot on, and the unfortunate reality is that many organisations are more vulnerable than they should be.
“We really haven’t done a good job addressing that problem, which is why ransomware is at record levels. Governments and industry need to work on new ways of approaching the problem - because existing approaches very clearly are not working - but, unfortunately, a quick fix is highly unlikely.”