BNZ's eftpos and ATM systems are working again, after a major outage.

Earlier on Saturday, the bank posted a Facebook message to customers alerting them to an "outage" which had left customers fuming as they found ATMs offline and payments refused.

It appeared to be a trans-Tasman outage, with customers of BNZ's parent bank National Australia Bank taking to social media to register their anger.

SUPPLIED BNZ chief executive Angie Mentis said it only had 0.7 complaints per 1000 customers.

In New Zealand anger flared on Facebook and Twitter.

READ MORE: BNZ 'ruthlessly vigilant' in deserving customer trust, chief executive says

BNZ customer Maxima Hughes said: "It has affected all your systems!, No internet banking, telephone banking, eftpos and ATM. Not nice when have to leave a full trolley for staff to return. It was an embarrassment!"

DAVID GRAY National Australia Bank owns BNZ.

Euan Russell said: "This happens more frequently than it should. How about actually spending some money on upgrading your Archaic DOS-based system so that it's reliable?"

Some customers were not happy with the bank's communications.

Reece Fallaize said: "When you found out about this problem why don't you text all of your customers??? No problem contacting me trying to flog me something!"

SUPPLIED All BNZ customers are all effected by a systems failure at the bank.

The bank has now apologised to customers, through Facebook.

"Cards and ATMs are now working as they should, and our other systems, including systems in our branch network, internet banking and mobile banking are following suit.

"BNZ apologises to all of its customers who were affected and inconvenienced by this, we appreciate the patience and understanding shown as we worked toward a resolution."

At 2.45pm, a bank spokesman said: "All systems will be working soon. It's just that there are pockets where systems will still be rebooting, resetting and coming back online."

Good morning, we're currently experiencing an outage that has affected some of our systems. The team is working to restore service as quickly as possible and we'll keep you updated on progress here. Apologies for the inconvenience and thanks for your patience. — Bank Of New Zealand (@BNZ) May 26, 2018

Some customers shared stories of the impact on their lives on Facebook.

Aimee Louise Smith said: "I drove all the way into Hamilton... to get some new clothes for a job interview on Monday. I'm now stuck because I can't access my funds to put more fuel in my car. I seriously hope you get this back online soon!"

Joe Hanita said: "How does a multi million dollar business not have a back up system for such a scenario ... this is incompetence in this digital age ... just finished eating breakfast when we get told we can't pay for it as BNZ systems are down ... thanks for letting us know ... nice way to get really embarrassed..."

Sieda Hornby posted: "Today is party day for our 6 year old and I'm not even able to purchase food and drinks or even pay the outstanding balance on the party place today."

Earlier this month BNZ reported a profit of $490 million for the half year to the end of March.

BNZ's new chief executive Angie Mentis said at the time the bank would be "ruthlessly vigilant" in ensuring customers' trust in it is well-placed.

She said BNZ received 0.7 complaints per 1000 customers, and the number of complaints escalated by the Banking Ombudsman was also low.

One BNZ worker, who asked not to be named, said the bank needed to invest more in upgrading the bank's systems.

Staff found the DOS-based systems slow and clunky, which made working at the bank frustrating.

Even newer systems, such as the system for making customer appointments had glitches, the worker said.

"I think their system could be much better. The upgraded ones we use are still very counter intuitive."

The Reserve Bank, which regulates banks, requires New Zealand banks owned by Australian banks to be able to operate separately from their parent banks.

But that only means there is the ability to separate them fast, not that they have to operate separately on a day to day basis.

The Reserve Bank had not been alerted to a systemic problem with the payments system, spokesman Angus Barclay said.

BNZ said: "Like most businesses, BNZ outsources some systems to third party providers. Current Reserve Bank outsourcing policy requires that all banks ensure that when services are provided by third parties including parent banks, that the appropriate recovery systems are in place for when failures occur."

"In today's incident, the robust backup systems in place with our parent bank meant we were able to limit the impact of this outage. Many BNZ customers were affected and we apologise for the inconvenience this incident has caused and thank our customers for their patience."