Hundreds of Newcastle Permanent customers across the Hunter region were left scrambling to pay for petrol, groceries and Father's Day purchases after a technical fault.
The issue caused debit card transactions to be processed twice.
It affected Visa debit transactions made on Thursday and Friday, with many customers waking on Father's Day morning to find their accounts overdrawn by hundreds of dollars.
Some reported balances pushed into the negative by more than $1000, while others said they were declined at checkouts, service stations and pharmacies because of the shortfall.
"Not a very nice Father's Day present," one customer wrote, describing waking up $146 in the red after doing her Father's Day shopping the day before.
Another said they were "declined trying to pay for a $4 drink" after their account was unexpectedly overdrawn.
Complaints flooded the bank's Facebook page and local community groups throughout Sunday as customers described double-ups on everything from supermarket shopping and fuel to streaming subscriptions, lottery tickets and medical bills.
Several said they had no other means of payment available and were unable to buy essentials including medication, until the issue was fixed.
Some customers also raised concerns about the flow-on costs of the glitch, including dishonour or overdrawn fees charged by third parties as a result of the doubled transactions.
Frustration mounted over the bank's limited weekend support.
"It's their responsibility," one wrote. "It should not be up to the customer to reach out to them."
Another said there were "no apologies, no follow-up."
"It's bad enough we were inconvenienced by this in the first place," one customer wrote, while another questioned why "management" weren't on hand on a Sunday to help resolve the issue.
Newcastle Permanent acknowledged the problem online on Sunday, confirming it was investigating an issue affecting Visa transactions.
It directed affected customers to its web chat, email or phone line for individual support.
As a mutual bank, its usual weekend contact channels were limited.
Phone and in-branch support resumed on Monday.
By Monday, a notice on the bank's website stated the debit card payments issue had been resolved and that affected accounts had been amended.
Customers requiring further support asked to contact the bank directly.
Several customers reported overnight that duplicate charges had already been reversed and their balances restored.
Newcastle Permanent was contacted for comment on the cause and scale of the outage, how many customers were affected, and whether the bank would cover any fees or charges incurred by customers as a result.
A Newcastle Permanent spokesperson said the organisation experienced a processing issue on the weekend that resulted in some transactions appearing twice in customers' accounts.
"This was limited to debit card transactions only," they said.
"The issue was identified on Sunday morning and resolved that evening.
"Both the issue and affected accounts have been rectified and customers' accounts are no longer impacted.
"As soon as the issue was identified updates were provided through our website, mobile app and internet banking channels.
"Extra staff are also available to assist customers.
"We're very sorry for the inconvenience our customers have experienced and thank them for their patience as we implemented a solution.
"We've now turned our attention to rectifying any Newcastle Permanent fees that were inadvertently applied to customer's accounts during this time and encourage any customers who have questions to get in touch with us through our webchat or by calling 13 19 87."
