A series of glitches affecting ANZ customers and merchants this year does not stem from the bank's technology merger with the National Bank, ANZ says.
The country's largest bank suffered intermittent outages to its Eftpos services on Sunday, leaving many shoppers around the country unable to make purchases.
Some customers later found they had been charged multiple times for the same transaction and many turning to social media for answers.
"My account was debited twice yesterday instead of once after the message 'cannot complete' came up when I went to pay by Eftpos," customer Ngaire Gow wrote on ANZ's Facebook page yesterday.
"I thought it may have been corrected over night but this has not happened and I would like my money back."
John Albertson, head of the New Zealand Retailers Association, said major retailers would have been badly affected by Sunday's outage.
"It's a damn nuisance to retailers and an inconvenience to customers," he said.
"I suspect some of the majors would be unhappy to say the least because we are very reliant on card transactions."
The problems on Sunday were not the first for ANZ this year.
In mid-April, a glitch caused overnight payments to be held up and denied ANZ customers of access to internet banking.
On January 31, a nationwide fault left many of the bank's customers unable to use their credit and direct debit cards over lunchtime.
There has been speculation that the ongoing issues at ANZ are a result of the bank's technology merger with National Bank.
ANZ has owned National since 2003 but announced last September that the two banks would be merged into one, under the ANZ branding.
As part of that merger, about 2.6 million ANZ customer accounts were transferred to National Bank IT systems in October.
ANZ today refuted any link between the merger and this year's run of glitches.
"These outages are totally unrelated to each other and to the ANZ-National Bank technology merger," a spokesman said.
"The Eftpos system is completely separate to the two core banking systems that were merged last year."
The issue over the weekend was caused by a hardware problem which was resolved on Sunday evening, the spokesman said.
Gull general manager David Bodger said the outage on Sunday was "disconcerting" and had caused major problems at the company's 14 unmanned service stations.
"If the customer gets declined at an unmanned service station, do they blame us, the bank or themselves? If they blame us and bother to ring us then we've got the chance to patch that up."
Many customers would simply blame Gull and head to another service station though, Bodger said.
He said Gull also been handling about one "ghost" transaction a month from ANZ, where a customer was debited twice after buying petrol, he said.
Albertson said it was up to retailers to take action with their bank if they lost out financially from glitches. He was not aware of any that were seeking redress from ANZ.
ANZ said it would be talking to affected customers on "a case-by-case basis".
The bank said whenever there was a technical issue, it identified the cause and looked at ways of minimising the chance of it happening again.