Downdetector, a website that logs internet outages, had more than 100 reports on the bank by 6.30pm yesterday.
At the time, the bank said it was aware of the problem and working to fix it as soon as possible.
One man - who did not want to be named - said he had just arrived from Australia and was shocked when he and his wife's debit cards all declined when they tried to use them at the airport.
"None of my cards would operate but, I checked my bank account details online, everything was saying zero. It looked as though it had been wiped out," he said.
Another woman came home from a dinner out in tears after having all her cards declined.
Numerous comments on Westpac's Facebook page had shared similar experiences on Friday night.
More than 12 hours later, the bank's Facebook posts wishing customers a happy Lunar New Year were pulled after comments criticising the bank's lack of updates on the problems affecting customers.
Replying to frustrated comments on its social media page, the bank said it was "really sorry".
"It's not fraud, it's a technical issue in which people's accounts are showing as zero balance.
"We're working hard to fix this. Sorry again!"
Shortly before 8am, the bank advised the situation had been resolved and apologised to all affected customers.