https://www.accc.gov.au/business/pricing-surcharging/payment-surcharges/qa-payment-surcharges Easy to read approach from the ACCC

https://www.accc.gov.au/media-release/excessive-payment-surcharge-ban
“The good news for consumers is that businesses can now only surcharge what it actually costs them to process card payments, including bank fees and terminal costs. For example, if a business’s cost of acceptance for Visa Credit is 1.5 per cent, consumers can only be charged a surcharge of 1.5 per cent on payments made using a Visa credit card,” ACCC Deputy Chair Dr Michael Schaper said.

The RBA indicated as a guide that the costs to merchants of accepting payment by debit cards is in the order of 0.5 per cent, by credit card 1-1.5 per cent and for American Express cards around 2-3 per cent. The ACCC has found that some merchants have incurred higher costs than these but any surcharge level imposed by merchants cannot be higher than the costs incurred by them for accepting that payment method.

https://www.legislation.gov.au/Details/C2016A00009 This is the legislation that relates to the fines for failing to comply with the RBA's legislation on Surcharge
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PAYMENT SYSTEMS (REGULATION) ACT 7998STANDARD NO. 3 OF 201.6SCHEME RULES RELAT|NG TO MERCHANT pRtCtNG FOR CREDTL DEBIT AND PREPAID CARD TRANSACTIONS This is the piece of legislation that we, the banks and any merchant attempting to manage their own surcharge need to read and comply with....happy reading ;)