Melbourne drivers are being overcharged about 4¢ a litre for petrol as stations bump up their retail margin, the consumer watchdog says.

The city's motorists have paid about $150 million too much for petrol in the past year, according to the Australian Competition and Consumer Commission.

​

"Even if you allow for cost increases petrol companies have incurred, the underlying price is still a good 4¢ a litre too much," ACCC chairman Rod Sims said.