The weakening Australian dollar is putting increasing pressure on petrol prices and will force motorists to pay more to fill up their cars in coming weeks.

Figures from the Australian Institute of Petroleum show the national average price of unleaded petrol fell by almost one cent to 143.6 cents per litre last week.

But CommSec chief economist Craig James says pump prices fell in only three of the eight capital cities - Brisbane, Sydney and Melbourne.

He says the falling dollar will lead to more expensive fuel.

"Certainly it's heading the wrong way for motorists and it's adding to more pain at the petrol pump," he said.

"Back in late July the Australian dollar was sitting at around about $US1.10 at that time.

"Now, if we had have held at those sorts of levels, motorists would be paying probably around about 13 cents less per litre or around about $8.50 [less] to fill up the tank with petrol.

"In Australian dollar terms, the Singapore gasoline price has risen by about 1 per cent, so it's fair to say that the low prices that we're seeing currently at the petrol pump are going to end over the next week to fortnight.

"Not substantially so, but we are going to see the upward drift continue at petrol pumps around the nation."

Mr James says there has been some extreme price volatility throughout the states and territories over the past fortnight.

"Canberra prices have risen by over 10 cents a litre - now you'd scratch your head to be able to find out some fundamental justification for that," he said.

"At the same time, Melbourne prices have been very volatile and in the last week they were down by four cents per litre.

"It could be the fact of competition, it could be other things at play, but it certainly would be good to get the ACCC to ask the question about what is behind the moves."