Saturday is now the most economical time to buy petrol although it is not likely to stay that way for long, said Australia's National Roads and Motorists' Association.

Over the past 10 months, the cheapest day of the week to fill up has changed seven times.

''We all got used to cheap Tuesday a couple of years ago,'' said NRMA Motoring and Services president Wendy Machin.

''But over the last 10 months it's really bounced around.

''That makes it impossible for motorists to get out there and buy their fuel at a good price.''

Ms Machin told reporters in Sydney there was no rational explanation why petrol prices were shifting, a scenario unique to Australia.

''It's something I've put to the oil companies, something I've put to the petrol commissioner, no one can give me a sensible answer,'' she said.

The petrol commissioner was established in 2008 to act as a watchdog and cease unfair behaviour in the market.

Ms Machin defended petrol commissioner Joe Damasi saying he often worked with the NRMA to investigate any unusual activity although his powers had to be strengthened in investigations and prosecutions.

''He could challenge the oil companies a bit more than he does and question some of the pricing information,'' she said.

''To date, as I'm aware, there has been no prosecutions.''

Mr Damasi, however, has raised the concern of the legal sharing of pricing information among the oil giants with the federal government.

''That gives them the edge we think,'' Ms Machin said, noting that petrol stations were able to access information that was unavailable to consumers unless they log onto often out of date internet websites.

''What we're saying is make the playing field level. Give the consumers the same sort of information that the industry has,'' Ms Machin said.

''Then they can make up their own minds about when they need to buy their petrol and when it suits their families needs and their budget.''