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Petrol prices rose to a near 29-month high in the past week, and motorists should prepare for fuel to rise by another four cents a litre in the next fortnight, an economist says.

The average Australian price of unleaded petrol rose by 2.9 cents a litre to 139.2 cents a litre in the week to March 6 - close to a 29-month high - data from the Australian Institute of Petroleum said on Monday.

Commsec economist Savanth Sebastian said motorists were likely to pay higher petrol prices in the near future after the national average price rose by 4.4 cents a litre in the past three weeks.

The political instability in the Middle East and North Africa is the key driver of near term prices, Mr Sebastian said in a statement today.

"And given the current tensions in the Middle East, the Singapore unleaded price has surged by over US$15 a barrel in the past three weeks and is holding at 30-month highs," he said.

"Unfortunately for motorists the Australian dollar can only do so much, and as such most of the increase in the global oil price will need to filter through to domestic pump prices."

Commsec forecasts petrol prices to increase by four cents a litre in the next two weeks to a national average around $1.44 a litre, Mr Sebastian said.

He said the cost of filling a family car was the biggest outlay Australian households make each week so a rise in petrol prices hits consumer spending.

"The average household is paying almost an additional $30 a month more on petrol compared with just over six months ago."