Petrol prices have been on the rise for the past two months.

Australian motorists have endured the sharpest new year petrol price rise in at least nine years, and analysts say further increases are likely this month.

Figures from the Australian Institute of Petroleum show the national average price of unleaded petrol rose 6 cents to 158.1 cents a litre in the week to January 5.

CommSec says it is the steepest increase to start a calendar year in records that go back to 2004.

The price jump is coming from a combination of rising gasoline prices in Singapore, which are near a four-month high of $US119.20 a barrel, and the depreciation of the Australian dollar over recent months which has made imports more expensive.

Wholesale prices at the terminal gate are at five-and-a-half-month highs of 148.3 cents a litre, up 14 cents over just the past two months.

These prices are up 2 cents over the past week, and CommSec expects that to be passed onto motorists within the next fortnight.

However, the good news is that the discounting cycle in most capital cities should hit its low point later this week before the price rises next week.

CommSec’s Savanth Sebastian says motorists should take advantage and fill up soon.

“By the end of this week, motorists in Sydney, Melbourne, Brisbane and Adelaide can buy fuel, with the help of shopper dockets, at or below cost price,” he noted in the report.

“However, prices are set to spike sharply higher by early next week, particularly across the eastern seaboard. The bottom line is motorists would be best served filling up the vehicle now, before prices spike higher.”

As of January 1, all new shopper dockets issued by supermarkets will be limited to a fuel discount of 4 cents a litre, due to an agreement between Coles and Woolworths and the Australian Competition and Consumer Commission.