Have you ever chosen a petrol station that seemingly offers the best bang for your buck, only to make the unpleasant discovery that the advertised price is valid only with a discount voucher or in-store purchase?

Canberra already has some of the highest petrol prices in the country, but changes announced today will stop the cost of fuel being understated by retailers that often advertise discounted prices prominently.

The changes would "help motorists make informed decisions about where they want to fill up," ACT Consumer Affairs Minister Shane Rattenbury said.

"We have ongoing concerns that some motorists are paying more at the bowser than they expected," Mr Rattenbury said.

"Consumers should not be lured into petrol stations by cheap prices which are conditional on having a docket or in-store purchase and have the right to know exactly how much they are going to pay at the pump."

As well as banning service stations from advertising fuel discounts, the new laws will force retailers to change the prices on fuel boards before altering those on the pump.

Mr Rattenbury said that, combined with an end of "confusing" advertising, meant motorists would never pay more at the pump than the prices advertised.

"This is about creating greater transparency in the fuel market, and making it fairer for all motorists who are trying to find the best deal at the bowser," he said.

"Motorists will be able to determine the real price quickly when driving past a fuel station."

Similar bans are already in place in Queensland, South Australia and Victoria — and ACT region director of the NRMA Kate Lundy said the move would help consumers.

"Motorists are sick of getting ripped off at the cash register because they think they are buying cheaper petrol than they really were," she said.

"The price that the service station is offering, is not the discount price, that's selective to those who are eligible, so it's a dishonest way of advertising."

Ms Lundy said the ACT was generally overcharged when it came to petrol, and any change that empowered consumers would be welcomed.

Legislation to make the change will be introduced tomorrow, and petrol stations will be given six months to remove the advertising once the laws are brought into force.