Petrol prices are increasing in the major cities, just weeks before a six-month fuel tax holiday expires.

Average prices across Sydney, Melbourne and Brisbane were up between 5.7 per cent to 9.4 per cent since September 12, Motormouth figures showed.

Average prices across those capitals were 166.3 cents per litre (cpl) on Sunday (up from 155.7 cpl on September 12) as servos increased the cost at bowsers after an unusually long trough in east-coast market cycles over the past two months.

Despite this, national average petrol prices fell slightly on the back of lower prices in smaller cities such as Adelaide and Darwin, where far fewer motorists live.

Prices are set to rise further in coming weeks before a fuel tax holiday ends on September 30, National Roads and Motorists Association spokesperson Peter Khoury said.

“Definitely fill up now,” Mr Khoury said.

From October onwards 25.3 cpl will be added to petrol prices as a six-month federal government fuel tax holiday expires, bringing delayed indexation with it.

Experts predict it will be a “double whammy” that will squeeze motorists at the pump against the backdrop of broader uncertainty across global oil markets.

Higher wholesale fuel costs

Petrol prices in major capitals bottomed out over the past week and then started increasing – delivering the biggest uplift in bowser charges since early August.

This came on the back of higher wholesale fuel costs, which rose 0.8 cpl last week to 152.3 cpl, according to the latest Australian Institute of Petroleum data.

Between September 12 and 18 average bowser prices in Sydney rose 14.9 cpl, prices in Melbourne were up 9 cpl and Brisbane saw prices increase by 8.7 cpl.

But these are just city-wide averages – the best way to find the cheapest prices is to use fuel tracking applications on your phone.

The latest rise in petrol prices comes after an extended period of falling bowser bills.

CommSec estimated the average family is paying $228.90 a month to fill up, below the record high of $297.90 during March.