Petrol cheapest on Saturday

SATURDAY is now the cheapest day to buy petrol but it's unlikely to stay that way for long, says the NRMA.

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

"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 said there was no rational reason why petrol prices were shifting, a situation 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 told reporters in Sydney on Friday.

The petrol commissioner was created in 2008 to act as a watchdog, stopping unfair behaviour in the market.

Ms Machin defended the role on Friday, saying petrol commissioner Joe Damasi often worked with the NRMA to investigate any unusual activity.

But his powers needed to be strengthened in investigations and prosecutions, she said.

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

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

But Mr Damasi has raised the issue of the legal sharing of pricing information amongst the major oil companies 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that petrol stations were able to access information that was unavailable to consumers unless they log onto often out of date internet websites.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."