A ban on excessive surcharging will be extended to include all businesses from the start of September, and this may force a re-think by small retailers that charge flat fees for customers who make small payments by card.

Under the new rules, small business owners must not charge any more in surcharges than what it costs them to accept payments made through EFTPOS, Visa, MasterCard or American Express cards issued by banks.

The exact cost of accepting card payments varies between banks and small business owners, meaning there is no single figure on what will be deemed "excessive."

But official figures show merchants typically pay their banks about 0.5 per cent of the transaction's value for debit card payments, 1 to 1.5 per cent for Visa and MasterCard payments, and 2 to 3 per cent for American Express cards.

"Our message to business is that you are not allowed to add on any of your own internal costs when calculating what surcharge you will charge customers," deputy chair of the Australian Competition and Consumer and Commission, Michael Schaper, said.

"The only costs businesses can include are external costs charged to you by your financial provider."

Big businesses have faced the same ban since last September, and the change prompted airlines to ditch fees of about $7 for paying for a domestic flight via credit card, instead moving to a percentage fee.

The ACCC is enforcing the ban and will be able to issue fines of $2,500 for sole traders that breach the rules, $12,000 for proprietary limited companies, or $120,000 for an ASX-listed company. For more serious breaches it can seek penalties of up to $1.3 million from a court. 

To help businesses figure out what is "excessive" banks have had to tell their business customers what they are being charged to accept credit card and debit payments in percentage terms.

Bessie Hassan of Finder.com suggested business owners check their bank statements, to avoid the threat of fines or damage to their reputations.

"If customers cotton on that you're charging more than you should, you could risk losing loyal customers," Ms Hassan said. 

The rule could cause changes for businesses that charge a flat fee for paying on card.

If a business charged an extra 10c for a $3.50 cup of coffee bought on credit, for example, that is equal to a 2.8 per cent surcharge, which may be excessive for the most common credit cards.

Dr Schaper said small business could still charge a flat fee, but they needed to make sure such surcharges were below their cost of accepting that payment in percentage terms.

Even so, he said there had not been a large number of complaints so far about surcharging among smaller businesses.

"The ACCC is not lying awake at night worrying about every coffee shop in the country, obviously our priority for these are large, systemic issues affecting large numbers of businesses. We expect small businesses to comply, but we understand that some of them are going to have to grapple with some of these issues," he said.

Taxis are not included in the surcharge ban because they are covered by state laws.