The Australian Competition and Consumer Commission says it's a lack of competition making Brisbane's petrol prices on average 3.3 cents per litre more than Sydney, Melbourne, Adelaide or Perth.
ACCC Chairman Rod Sims said Brisbane motorists "suspected" they were paying more, and now the report backs it up.
"Some local fuel retailers are enjoying high profit margins at their expense," Sims said.
In 2015–16 the average net profit per fuel retailer was around 55 per cent higher than the Australian average.
The ACCC says they could get away with this due to "weak retail competition".
"The high retail prices and margins for petrol in Brisbane have cost motorists there around $50 million per year more than their interstate counterparts since 2009–10. This is despite wholesale prices in Brisbane being similar to those in the other four largest cities," Mr Sims said.