The NRMA in New South Wales is urging the consumer watchdog to investigate why it has been difficult to predict the petrol pricing cycle over the past few months.

Tuesdays were the cheapest day of the week to buy petrol, and motorists knew to fill up before prices went up again for the rest of the week. But since May this has changed.

NRMA spokesman Peter Khoury says the NSW branch has been concerned about the volatility of the cycles in capital cities for some time.

He wants the Australian Competition and Consumer Commission (ACCC) to investigate this volatility.

"Once upon a time motorists knew that if they pulled into a service station on a Tuesday, for example, then that would be the day they could get cheap petrol as part of this weekly cycle," he said.

"However now for some time the cycle has shifted on an almost weekly basis.

"It has become impossible for motorists to be able to predict when the best time to buy petrol is, and it wasn't always this way."

Mr Khoury says NRMA expects petrol prices to remain around $1.34 to $1.35 on average and expects a considerable jump tonight.

He says prices are expected to peak tomorrow around $1.47 on average.

But despite being able to predict the one week, Mr Khoury says this pattern does not continue in following weeks as it used to.

"That of course is a cycle - [it] does what it's been doing for the last couple of weeks," he said.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Listen Duration: 3 minutes 23 seconds 3 m 23 s Calls for more predictable petrol price cycles ( Hagar Cohen ) Download 1.6 MB

"However it is a volatile cycle now and it's difficult to actually be able to forecast because it changes so frequently."

Mr Khoury says the public would like to know what has brought about this change.

"We certainly don't have that sort of information, it's something that we've raised a number of times and I know that the ACCC acknowledged that this situation is occurring in its most recent petrol report," he said.

"But I think the community would like a better understanding and a bit more transparency in the market as to why this has occurred.

"Because there's nothing more frustrating than when you get in the car to go to work in the morning and the price is 15 cents a litre dearer when you get in the car and drive past the service station on your way home on the same day."

In a statement, the ACCC said it expects average post-Christmas petrol prices to be relatively stable due to stability in the international benchmark price.

"Across most of Australia's largest capital cities where petrol price cycles operate we have seen the cycle extend to around 10 days in duration," the statement said.

"In Melbourne, Sydney and Brisbane the petrol price cycle began to fall before Christmas. This may mean prices will rise due to the price cycle mid-week."