Telecommunications companies are misleading customers over broadband internet speeds and the worst offenders will likely face prosecution over dodgy advertising by the end of the year, the consumer watchdog says.

Chairman Rod Sims said the Australian Competition and Consumer Commission (ACCC) would conduct "compliance sweeps" of broadband marketing and telco websites later this year in a bid to keep telcos honest about speeds available on the national broadband network.

"Right now, consumers are not getting the basic information they need to make an informed choice. Indeed, they are often being misled," Mr Sims said on Thursday.

"We want to see consumers presented with information based on the realistic speeds they can expect to experience, particularly during busy periods. Not just best-case scenarios."