Banks are contacting hundreds of people who were exposed to an ATM fraud where cash-card details were copied and used to rob accounts of more than $20,000.
It is New Zealand's first case of "skimming", where sophisticated devices are used to harvest account information from cards while a small camera captures the Pin number.
The fraud has been traced to a Bank of New Zealand automatic teller machine in Great North Rd, New Lynn.
BNZ has taken the unusual step of suspending 1300 credit and debit cards that were used at the machine between March 7 and 28.
A further 700 cards from other banks were put through the machine during the three-week period.
BNZ said 21 customers had been targeted and around $20,000 was taken.
Two ASB customers lost a total of between $3000 and $5000.
Banks say they will reimburse the money.
BNZ confirmed that it was alerted to a possible fraud at the New Lynn machine more than two weeks ago.
Shona Bishop, BNZ general manager marketing and business development, said a customer noticed something suspicious attached to the ATM and contacted the bank.
Other customers notified the bank of "unusual transactions".
"We immediately, a few weeks ago, contacted the other banks and closed down those accounts and stepped up the surveillance. No further activity happened on those accounts or those cards until [Tuesday]."
BNZ had been physically monitoring the machine but did not find any skimming device, she said.
"There are multiple types of skimming devices but you would always normally see them, however some customers would not notice them."
The bank has laid a complaint with police.
Ross McEwan, ASB head of retail banking and marketing, said two customers had lost a combined amount of between $3000 and $5000.
He said 160 ASB cardholders had used the ATM during the period.
Other banks were contacting customers but said no one had reported missing funds.
Westpac said 115 cardholders were being contacted and National Bank said it had stopped the cards of about 100 customers who were affected.
ANZ would not say how many customers were affected but said it had stopped their cards and would issue new ones.
A spokesman for Kiwibank said BNZ contacted the bank at least 10 days ago to voice concerns about a possible skimming scam.
Ms Bishop said a customer "thought they saw something on the machinery" two weeks ago.
"We absolutely did everything, including working with our banking colleagues to say, 'Hey guys, you watch out as well but we can't see anything that is happening yet.'
"We didn't put customers at risk at all. We stepped up our surveillance and the moment we saw our customers were at risk we suspended all 1300 cards and straight away went to the other banks."
There are an average of 250 transactions a day at each of BNZ's 450 money machines and a total of 2300 ATMs in operation around the country.
Ms Bishop said skimming was a major issue for the banking industry.
"This is the start of a new wave of quite serious criminal intention."
David Russell of the Consumer Institute said skimming devices could be bought on the internet for as little as $50.
He said skimming at ATM machines could involve a number of techniques to harvest Pins, from cameras installed at the machines, mirrors and people standing behind the user to note down the code.
Skimming is common overseas and is often carried out by organised gangs.
Two skimming schemes have been cracked by Australian police in recent months. One group was operating in Sydney in September and four men were arrested on the Gold Coast this month.
Australia's first known skimming case robbed 315 Sydney customers of more than $700,000 in 2003.
How skimming works
1. A lookalike device is fitted over the existing card slot to read card details.
2. A disguised camera is placed near the slot to photograph the keypad and PIN number.
3. Devices containing stolen card and PIN number information are removed.
Cash machine 'skimmers' steal thousands
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