Kiwibank's systems are up and running again after a two-hour outage which struck around midday, in the middle of Labour Day sales.
Customers were unable to withdraw cash from ATMs and their EFTPOS cards were declined in shops.
"Pretty embarrassing when at checkout and can't buy anything especially when you have a baby with you who doesn't want to sit around," one customer wrote on Kiwibank's Facebook page.
Another said it was embarrassing having to put their pie, chicken roll and drink back at the bakery, where there was a long line.
Kiwibank spokesman Bruce Thompson said everything was fixed around 2pm.
"Our system was quite badly affected between 12 o'clock and 2 o'clock, and we had to do a work around whereby we transferred our IT systems from Auckland to Wellington," he said.
"Everything is now fully functional and we apologise for the inconvenience."