CommSec says petrol prices are set to head toward three-and-a-half-year highs by the end of the week.

The Australian Institute of Petroleum says the national average price of unleaded petrol rose one cent a litre to 151.6 cents last week.

Gains in the Singapore Tapis crude price and tension over Iran's nuclear program are expected to continue to drive up fuel prices locally.

CommSec chief economist Craig James says motorists should fill up before the week is over.

"It's certainly a case of picking your time right and most of the mainland capital cities that have discounting cycles were at the low point at present and that doesn't usually happen around a holiday period," he said.

"So motorists should be taking advantage of the opportunities that present themselves to fill up the petrol at the cheapest price.

"Once we see petrol prices move up, the peak price is probably going to be somewhere between $1.55 and $1.60, so it's getting up to the highest levels that we've seen in around three-and-a-half years."