Russia's invasion of Ukraine is partly to blame for inflated diesel prices, which have soared beyond the cost of regular petrol.
The price of diesel remained significantly higher than petrol in the December quarter of 2022, with supplies reduced from Russia as well as from France because of refinery strikes.
Diesel prices were also kept high in the quarter due to increased demand for heating during winter in the northern hemisphere, the consumer watchdog found.
Diesel set people back an average of about $2.23 per litre in Australia's largest capital cities of Sydney, Melbourne, Brisbane, Adelaide and Perth during the quarter.
That was about 40 cents more per litre than average petrol prices, which came in at about $1.83 per litre in Australia's largest capital cities, the Australian Competition and Consumer Commission found.
"Our report shows a range of international factors contributed to higher retail diesel prices in Australia," commission chair Gina Cass-Gottlieb said.
"The small increase in average retail petrol prices in the major cities reflects the increase in taxes following the restoration of the full rate of excise from 29 September 2022, offset by a decline in international petrol prices."
Australia's fuel prices are largely determined by international refined fuel prices.
International refined fuel prices dipped in the December quarter, but the differences between the cost of diesel and petrol remained after Russia's invasion of Ukraine prompted a rise in diesel costs in early 2022, Ms Cass-Gottlieb said.
Crude oil prices also continued to drop in December on concerns about a potential global recession, she said.
But a fully restored fuel excise and a weakened Australia-US exchange rate meant average petrol retail prices increased on the previous quarter.
Average petrol prices were highest in Melbourne among the largest capital cities, sitting at about $1.85 per litre, while they were lowest among the big cities in Adelaide at about $1.79 per litre.
They dropped in Canberra and Darwin by about seven cents per litre compared with the previous quarter, and rose in Hobart by about three cents per litre.
The average price of petrol in the regional locations the watchdog monitored remained steady at $1.87 per litre.
The consumer watchdog welcomed NSW's November 2022 expansion of its fuel price transparency scheme into the ACT, along with the Northern Territory government's announcement it would start publishing historical fuel price data.
Victoria was Australia's only remaining jurisdiction without a real-time and comprehensive fuel transparency scheme, the watchdog noted.
