Kiwibank customers are outraged over technical issues with its core banking services, some saying they can't get money out, make payments or check their account balance.
In an update posted on its Facebook page just after 11am on Monday, Kiwibank said it was having "continued intermittent issues" providing services to customers.
Affected services including internet banking, its app, website and phone services.
"We've identified the cause and are working hard to fix the issue," Kiwibank said in a statement.
Apologising to customers, the bank said "continuity of service" is it's priority.
"We apologise to our customers for any inconvenience caused and thank them for their patience and support," the statement said.
Kiwibank has received a flood of complaints from customers in response the post.
One woman said she couldn't get cash out of an ATM, another responded that the machine had "stolen" her card.
A couple of customers confirmed they could use EFTPOS, some said the Kiwibank app wasn't working.
"It almost feels like we have been robbed of our accounts. We can't log in both website and online app. Definitely a worry for sure," one person said.
Others were frustrated they couldn't pay for groceries.
"I just missed today's last delivery spot for my groceries and now my Granddaughter will not have food until tomorrow… if you've even fixed it by then," another said.
One man said he had tried to get through to the call centre but had been cut off six times.
"I waited for over an hour on the phone to speak to someone and gave up!!! SLACK!! (after being cut off six times trying to call you!)"
But others called for calm and asked that the bank be given a chance to fix the issue.
"It's been up and down this morning. Thank you Kiwibank for looking into the issues to be able to fix the issues. I wish people would understand technical issues happen from time-to-time," another said.
Asked for an update on the issue and when customers could expect it to be resolved, a Kiwibank spokeswoman said the cause had been identified but did not provide further details.
"We do know what the cause is and our tech team is working to fix it," the spokeswoman said.
"The priority for us at the moment is continuity of service for our customers."
Kiwibank customers experiencing difficulties logging into internet banking are advised to "try back later".
Customers would be updated via Kiwibank social media channels, the spokeswoman said.
In an update posted to the Kiwibank Facebook page at 5:25pm, the bank said teams were still working through the fix. It didn't have an estimated completion time.
"This is likely to continue into the evening."
ATMs and cards can still be used and automatic payments are unaffected, the bank said.
"Our contact centre is experiencing higher demand and our team is working through the calls as best as they can."