Collapsing world oil prices have delivered most Australian motorists a 20 per cent reduction in fuel prices since the middle of the year.

But depending on where you live, the benefits at the bowser vary considerably.

The average cost of petrol in Australian capital cities was $1.23 a litre on December 21 – almost 30 cents lower than at the beginning of June.

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But price data collated by the Australian Institute of Petroleum (AIP) indicates that petrol retailers operating outside the major capitals are yet to pass on the full benefit of lower oil costs.

Adelaide, Melbourne and Sydney have cheapest fuel

The AIP market data shows that Adelaideans enjoy the cheapest fuel in the country.

In the week ending December 21, the average price for a litre of petrol in the City of Churches fell almost 8 cents to $1.16.

Melbourne drivers were the next best off, paying around $1.20 a litre, while Sydneysiders paid a cent more.

Darwin is still the most expensive city to fill up a tank.

The average price last week in Darwin was $1.49 a litre – 23 cents lower than the year high average price of $1.73 recorded in early October.

In Hobart, the average bowser price has shed 19 cents to $1.42 since June.

It appears that Canberrans are getting the worst deal.

Since June, the average price in the nation’s capital has only dropped 12 cent to $1.44.

National Roads and Motorists’ Association director Alan Evans said the difference between the average unleaded price in Sydney and Canberra was “preposterous” and could not be justified.

“The supermarkets and their associated oil companies should be Santa Claus and not Scrooge to Canberra motorists,” Mr Evans said.

“A large supermarket chain has capped prices in the Sydney metropolitan area at $1.14 or less until January 4 next year, while independent retailers are offering regular unleaded in Sydney as low as $1.12 per litre.

“We would like to see the petrol companies extend this Christmas gift to the Canberra market.

“Canberra needs more competition though and the average price should fall to under $1.20 per litre.”

Regional centres dudded again

It appears that the big petrol retailers are refusing to pass on the full impact of the oil price plunge to motorists that live in regional areas.

According to the AIP data, the average price ($1.38) being paid in regional centres is only 16 cents lower than in June.

In some parts of northern New South Wales, motorists are being scalped.

Port Macquarie residents paid $1.48 a litre last week.

Petrol retailers are simply fattening their profit margins in regional parts of New South Wales, according to the NRMA.

“North Coast motorists deserve better and have a right to feel incensed by the gap between local prices and Sydney prices,” said NRMA deputy president, Wendy Machin.

Stroke of luck for Joe Hockey

While the price slide is a blessing for households struggling to keep up with soaring water and power bills, it might also hand Treasurer Joe Hockey a “get out of jail card” on his controversial decision to increase the excise tax on imported oil.

From next February the tariff on imported oil will rise every six months by a percentage equal to the movement in the Consumer Price Index.

Hockey’s tweaking of the tariff is expected to rake in additional revenue of more than $19 billion in the next decade.

Although minor party senators have indicated they will oppose the increase, the new tax might be a tad easier for the Treasurer to sell to electors if wholesale oil prices continue to plummet.