The consumer watchdog said at the "extreme end" it had received reports of RATs costing up to $500 for two tests through online marketplaces, and more than $70 per test through convenience stores, service stations and independent supermarkets.
Despite wholesale RAT costs being up to $11.45 a test, the agency said prices for the kits are often retailing between $20-$30 and are priced much higher through smaller outlets.
"It's just beyond outrageous ... it's extremely concerning," Mr Sims told reporters in Sydney on Monday.
He labelled retail mark-ups of more than 100 per cent on RATs as "beyond the pale" and said the agency was working with the Therapeutics Goods Administration and the Australian Federal Police to stamp out unscrupulous selling of RATs.
"We're very much looking forward to what (retailers') explanations are for the very high pricing that's been reported," he said.
"Often some of these high prices are at stores you wouldn't expect like petrol retailers, tobacconists and convenience stores. We're very much looking at those as well as pharmacies."
In addition to exorbitant prices, some retailers were failing to provide receipts, while other traders were breaking up wholesale bundles of RATs designed for medical centres and selling them retail.
The agency had also received reports of people onselling RATs at more than 20 per cent mark-up, which was illegal.
Mr Sims said the ACCC planned "very soon" to take action - which could include fines and legal proceedings - against profiteering retailers.
"There's a lot going on and we hope companies hear this message and adjust their behaviour," he said.
The watchdog said the "concerning practices" had come to light from analysis of more than 1800 reports from the public since Christmas.
Close to 150 reports are coming in each day from concerned members of the public about sky-high RAT pricing, it said.
Chemists are the worst offenders, followed by convenience stores, tobacconists, supermarkets and petrol stations, with the ACCC pointing in particular to some King of the Pack and Metro Petroleum stores.
The ACCC has so far contacted more than 40 test suppliers, major retailers and pharmacy chains across the country reminding them they need to back up claims to consumers about reasons for higher prices.
"We will continue our investigations and analysis of information from consumers, retailers and suppliers, and will provide further updates in coming weeks," Mr Sims said.
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