Australians should shop around if they want cheaper petrol, the head of the competition watchdog says.

Prime Minister Scott Morrison put the Australian Competition and Consumer Commission on notice over rising fuel costs this week, saying he is “expecting action”.

But ACCC chair Rod Sims says Australians who want to pay less for petrol should work out the best times and places to fill up.

“What consumers can do is shop around. I know it sounds trite, but there is money to be made,” he told ABC Radio on Wednesday.

Mr Sims said the ACCC would continue to closely scrutinise the petrol market and take legal action to address illegal behaviour, as well as advise Australians on how they can save cash on fuel.

But he said most of what people pay is dictated by overseas petrol prices and tax.

“Before you even get to the local service station, 85 per cent of the price is explainable by things that are very hard to do anything with,” he said.

Divestment powers are also not the solution to improving the petrol market in his view. 

The cost of petrol has been rising across the nation, with prices in Melbourne hitting a 10-year high last month, and Adelaide motorists forking out an average of $1.67 a litre as of Monday.

Mr Morrison earlier this week said he wants answers from the ACCC.

“They’re the cop on the beat, they’re the ones we fund to go out there and monitor why prices are moving up and down and how they’re timed,” he said on Tuesday.

The prime minister has conceded some factors, such as the global cost of oil, are out of the local market’s control.

But the ACCC has powers to investigate if consumers are being ripped off.

“What we can ensure is the people who are selling it here behave,” Mr Sims said.

-AAP