The falling Australian dollar and higher global oil prices are being blamed for hurting motorists at the petrol pump.

Prices averaged 158.6¢ a litre in Canberra on Monday, according to the NRMA, while in Sydney the average price was 161.6¢ a litre.

A spokesperson for the NRMA said it was very unusual for Sydney prices to be higher than the ACT's.

''Normally there are consistently higher prices in Canberra because there's less competition. Independents play a huge part in driving that price down, and unfortunately Canberra doesn't have that,'' he said.

CommSec economist Savanth Sebastian said the cost had soared off the ''double whammy'' of the dollar and Singapore prices, which were at four-month highs of about $120 a barrel.

Mr Sebastian said prices would ease over the next week, but continue rising during the peak periods of the retail cycle.

''Over that early January period we'll probably see fuel prices over the next fortnight jump another two or three cents a litre,'' he said.

But Fueltrac's Geoff Trotter said higher global prices following instability in Libya, South Sudan and Syria accounted for a rise of only a few cents a litre. Mr Trotter blamed Coles and Woolworths for artificially inflating prices to offset the cost of their discount shopper dockets.

The supermarket chains have given an undertaking to the ACCC to wind back their fuel discount offers to supermarket customers to 4¢ a litre from January 1. But both companies rejected Mr Trotter's claims.

''Petrol prices have started moving up, not at our instigation, and generally reflect higher oil prices internationally and the softening Australian dollar,'' a Woolworths spokeswoman said.

A Coles spokesman said: ''The increase in pump prices is the result of a range of global factors and something fuel retailers across the country are having to face.''

with Ben Westcott