The federal government will take action if drivers are being ripped off at the petrol bowser, the treasurer says.
Australian Competition and Consumer Commission chair Rod Sims believes drivers are paying three cents a litre more than usual.
"In our view, margins are two to three cents a litre too high," Mr Sims told 3AW Melbourne radio on Monday.
"That's $400-600 million - that's a lot of money for Australian motorists."
Mr Sims said it's not against the law for consumers to be overcharged for goods, but motorists can work out the best time to buy based on price fluctuations.
"When prices are at bottom of the cycle you're probably getting petrol below cost and when they're at the top of the cycle you're getting ripped off big time," he said.
"It just fluctuates in a way that irritates the hell out of motorists, but it's not against the law the way that's happening."
Treasurer Josh Frydenberg said the government would consider any suggestions by the consumer watchdog to push down petrol prices after the cost of premium petrol hit $1.90 a litre in the past week.
"If they make any recommendations to government then, of course, we will favourably consider those," he told Nine Network.
"What we understand about our petrol market is that it is driven to some extent by what happens overseas, with the Australian dollar, but also with global oil prices.
"But if companies here are ripping off consumers then we will take action."
Mr Sims says 35 per cent of the cost of petrol is tax and Australians are paying a fuel excise of 41 cents on top of the GST.
The tax is meant to fund roads but he says it's questionable whether the taxation level is appropriate.
"The link is so poor, you have no idea whether this level of taxation is the right one to fund the roads," he said.
"If we could just try and get some better road user charging, so motorists know as they as they pay money, it's actually going to build roads and they're not paying more than they should, that'd be a good step. That's 85 per cent of it."
Australian Associated Press