People are being urged to do research on websites before they shop online as two major banks report a surge in complaints about credit-card fraud and transaction disputes.

Netsafe chief executive Martin Cocker said there had been a steady rise in scams and fraud associated with online shopping.

"Consumers are looking to buy online and spending more money there and creating more opportunities for criminals."

As people made more online transactions it became more difficult for banks to monitor spending and identify unusual purchases.

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"They do cop the cost of a lot of online fraud back into their business so they've got a real incentive to combat online fraud."

Netsafe provides a guide for shopping safely online, including only using trusted Wi-Fi connections, strong passwords and a credit card or well-respected payment service.

Netsafe also has advice on what to do if something goes wrong, including talking to your bank and applying to the Disputes Tribunal.

Cocker said people needed to be careful about the businesses they were purchasing from and do some research.

"There is nothing to say that a fraudulent website, shop site, looks any different from a legitimate one."

ASB said it had been experiencing an "unprecedented increase" in dispute inquiries in recent months.

An ASB spokeswoman said the bank had experienced a steady increase in disputed transactions during the past few months and put it down to the rise in online activity.

"The recent increase is due to a number of reasons, driven mainly by an increase in the volume of online transactions, as more people embrace making digital and online purchases."

As well as more fraudulent transactions, the bank was seeing more customers disputing charges for transactions such as online games, subscriptions they were not aware of, or when a free subscription time had expired and the customer had been caught out.

ANZ, the country's largest bank, said it received a lot of queries through December and January and had to add an extra 10 staff to ensure it met its 21-day dispute promise to customers.

"ANZ recommends customers research the companies they are buying from overseas.

"Customers should ensure they read the fine print on free trials and subscriptions and be aware of lending devices to people which have card details stored to purchase online games.

"Customers should also be alert to phishing attacks and avoid responding to SMS or email messages purporting to be from their bank, or other trusted entity."

New Zealand Bankers' Association deputy chief executive Antony Buick-Constable said it was strongly recommended that people check out online sales websites before making purchases.

"If you haven't used the site before, or aren't sure they're a reputable business, it pays to do some research."

In the case of online subscriptions people should make sure they knew what they were signing up to.

"Often a quick online search will help you find out if the site you're dealing with is reputable," Buick-Constable said.

The association provided a guide to help people protect themselves from online scams.

People who thought they had been taken in by a scam should contact their bank immediately.

"The sooner they know, the more likely they'll be able to help you," he said.

"For example, if you don't receive the goods you've purchased by credit card, or if they're faulty, you can request a chargeback and have your purchase refunded.

"Before purchasing anything online, make sure you're dealing with a business you can trust."