Customers of the Bank of New Zealand are threatening to move their business elsewhere after issues with its online banking system have dragged on for five days.

But the bank says the problems are mostly resolved now with only one area still to fix - a delay in the data feeds from the bank to accounting software providers Xero and MYOB.

More than 400 comments have been posted to the BNZ's Facebook page since Monday when it began experiencing problems with access to its online and mobile banking services.

The bank initially said it had resolved the problems by Monday afternoon but on Tuesday it had problems with account balances showing and on Wednesday it was back to its online banking system being down.

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By 9.50pm last night the BNZ said all internet banking services were available but it was continuing to monitor the situation.

This morning BNZ said that following restoration work overnight full service functionality had been restored to its internet and mobile internet banking.

But the processing of customer data feeds for Xero and MYOB continued to run behind by one or two days.

Customers have vented their frustration online.

Kayla Lawton yesterday said it was the fourth time internet banking and the app was down.

"One more chance BNZ before I move my entire banking history etc including my mortgage to another provider."

Eamonn James said he would be switching banks.

"I haven't been able to check if payments have been made to me due to there being an outage for almost the entire week. So ridiculous."

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Phillip Rusbatch posted that it was time to look at another bank.

"Can't pay any bills and about to get pissed off people at me because I can't pay them online. Do we have an eta on this please? I've been banking with you folk for over 20 years and I'm about to go elsewhere."

The threats to depart were met with apologies from a BNZ staffer who said the service was "completely unacceptable" and "definitely not the service you should expect from BNZ".

Other customers complained of not being able to transfer money to pay for groceries or to other people with one woman frustrated she couldn't send money to her daughter to pay for petrol to allow her to get home.

Some were also worried about the security of their money.

Leyton Smith said he had been a customer of bnz for almost 30 years and wanted an explanation for the problems quickly.

"This is absolutely silly, how can we trust our money is safe with bnz when we can't even access it! I will be dropping in the branch tomorrow for an explanation."

The bank responded by assuring the customer security of their money was paramount to the bank.

"We take this very seriously."

It is not the first major outage the BNZ has had.

In June last year the bank's online banking, eftpos, credit card, ATM and branch services went down on a Saturday for several hours cutting people off from accessing their accounts and prompting hundreds of customers to post complaints on the BNZ Facebook page.

The outage was blamed on a power failure in Australia affecting the data centre run by its parent company National Australia Bank.

A BNZ spokesman said the latest issues were unrelated to the power problem last year and were caused by issue within its server environment and how some elements were talking to each other.

"From Tuesday there were some intermittent issues with online banking where some transaction information wasn't available for customers in the app or online. It's now resolved."

The spokesman said as part of the fix it had to restart some of its internet banking services, which caused some periods of downtime.

"We're working with accounting software providers to catch up their bank feeds for business customers, and we expect the last of these transactions to be processed today."

Throughout the period ATMs and card transactions were unaffected.

"We apologise to all of our customers for any inconvenience caused."

He said it encouraged anyone who believes they have been impacted by the outages should get in touch with the bank to discuss their situation.