PETROL prices are expected to soar to as high as $1.67 a litre as thousands of motorists return from holidays.

Victorians have already endured an expensive start to the new year with Melbourne's unleaded peak hitting 164.9c last Thursday, the steepest since July last year, before a steady retreat.

Drivers are urged to fill up ahead of another likely 2c-a-litre retail price rise late next week.

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CommSec's Savanth Sebastian said a combined international cost surge and weakening Aussie dollar had increased wholesale prices that were due to be passed on at the pump.

"It has been a double whammy since mid-November and in the short-term motorists should brace for more pain," Mr Sebastian said.

"But there are signs there could be a little bit of a reprieve of 1 to 2c in a few weeks."

Melbourne's unleaded costs on Wednesday spanned 144.9c to 160c a litre, according to the RACV.

The afternoon average was 152.1c - almost 13c more than last January.

Country averages ranged from 149.9c in Bairnsdale to 162.1c in Corryong.

RACV vehicle engineering manager Michael Case said recent price pressure had crashed with stretched budgets after Christmas.

Motorists should try to buy at or near the low point of the metropolitan price cycle, currently about $1.45 to $1.50 a litre.

Based on recent patterns, prices could peak again from next weekend.

"However, there is no guarantee of timing as the cycle has been unpredictable."

Mr Case said Melbourne's unleaded prices reached a record $1.69 a litre in July 2008.

Regional areas usually did not experience the wild peak and trough fluctuations of the city.

Country price differences were mainly due to varying degrees of local competition and fuel transport costs.

Mr Sebastian said the Singapore unleaded price rocketed from $118 to $133 a barrel from mid-November to late last week, before a slight easing.

Marcus Worrall, 40, of South Melbourne, was stung having just had two family driving holidays to Portsea and Anglesea.

"I think they deliberately put it up over holidays because they know people are going away," he said.

"But when you have to top up, you have to top up. There's not much you can do about it."

karen.collier@news.com.au