Auckland Transport has backed down and made changes to a bug that freezes money on HOP cards for about 16,000 users.

Following public and political criticism, Mayor Phil Goff said AT has come up with a "fair outcome" to the HOP card rule that sees balances disappear if people top-up their HOP card online and do not tag on within 60 days.

When Goff raised concerns about the bug two weeks ago, AT dug in its heels and refused to fix the problem. A day later, Goff and senior councillors gave AT bosses a sharp reminder they are there to serve the people.

Goff says the outcome will see AT put an online warning up front telling people they need to tag on within 60 days.

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AT had also agreed to put a system in place for people who do not tag on within 60 days to get an automatic refund back to their bank account and an email confirming the refund, he said.

"That solves the problem of people inadvertently losing their money," Goff said.

The third measure AT was looking to introduce, Goff said, was a credit card or mobile phone system to tag on and off. That would be useful for casual users and could be in place within 18 months.

Goff said he was happy AT had responded constructively, saying chairman Lester Levy had bent over backwards to the concerns and said the customer is always right.

AT has said online top ups are done on 15 per cent of HOP cards and 97.4 per cent of those tag on within 60 days.