For a long time, Simon* has been extremely careful with his personal information, shredding bills, not signing up to random offers, and sticking with one bank.

So he was surprised when a rival bank, ANZ, sent a personalised letter to his home, inviting him to become a "platinum member".

After some hassling on the phone, an ANZ representative revealed it had received his details from Veda, Australia's largest credit reporting agency that is mired in controversy.

"I got the letter a few weeks after I bought my personal credit report from Veda which I needed for a new contract," said Simon, an IT consultant from Melbourne.

"I feel my privacy's been breached, but I did fill out the form in a rush because I was under pressure, so they may have gotten my consent without me realising."

Privacy advocates have slammed Veda for using the online form to obtain a free or paid-for credit report - every Australian can by law access it for free once a year - as a "marketing opportunity".

It appears Simon left a pre-ticked box at the end of the online form untouched, inadvertently giving his consent to Veda and its "corporate partners" to use his personal data for marketing purposes.

It is illegal for Veda to sell credit reporting information for marketing purposes. But it is a powerful player in the marketing sector, flogging other personal information to banks and other businesses.

Justin Malbon, a Professor of Law at Monash University, said it was highly likely Veda knew a pre-ticked, opt-out box would lead to a higher rate of people giving their consent.

"Many behavioural studies show there's an inertia and a 'not noticing', leading to more people leaving it ticked and giving their consent," he said.

"If you turned the other way and asked people to opt-in, you're going to get far less people."

A Veda spokeswoman said concerned consumers could get in touch and take back their consent.

"A person ordering Veda products is clearly presented with information, including the option to un-tick any box," she said.

"Veda's approach is in accordance with its obligations under the Privacy Act and the Australian Privacy Principles."

Veda also said it did not use credit reporting information in any way to calculate and determine whether a person was a suitable target for a client.

While a degree of consumer responsibility is involved, Fairfax Media understands the box wasn't pre-ticked a couple of years ago.

A consumer can access their credit report once a year for free, with the Privacy Act saying Australians have the right to obtain a correction for any inaccurate, out-of-date, incomplete, irrelevant or misleading information.

Kat Lane, principal solicitor at the Financial Rights Legal Centre, has demanded Veda to remove the pre-ticked box altogether, especially on the form to obtain a free credit report.

"You're getting your free credit report that we're allowed once every 12 months under the law, and they're using it as a marketing opportunity," she said.

"How on earth is it relevant to getting your free credit report? That's just misleading."

An ANZ spokesman confirmed that, like the rest of the banking industry, it uses the marketing services of Veda to target potential customers.

"ANZ requires assurances from third parties that the source of information and the marketing activities that information is used for are entirely compliant with privacy legislation," he said.

Fairfax Media recently revealed NAB was being tipped off by Veda each time a business banking customer approached a rival about a loan, sparking fears about privacy breaches and unfair competition.

In response, Opposition leader Bill Shorten said evidence was growing everyday showing the need for a royal commission into the banks - which the Coalition has dismissed.

Fairfax Media contacted 15 large and small banks, and 11 said they were not engaged in the same monitoring practice.

Credit Suisse and Macquarie Bank declined to comment and Suncorp did not respond.

Veda was snapped up by US credit reporting behemoth Equifax for $2.5 billion earlier this year.