Optus has been ordered to pay a fine of $5.26 million after losing a case about misleading advertising.

The Australian Competition and Consumer Commission sued the company, formally known as Singtel Optus, in the Federal Court over an advertising campaign for broadband internet plans launched on Anzac Day last year.

In a judgment published today, the court ruled there was an "essential vice" in the advertisements, because they misled people about the download allowance they could get under the plans.

"They suggested that a consumer would obtain a broadband usage allowance of 120GB or 150GB [depending on the particular plan] consisting of two usages allowances - one peak, one off-peak," Justice Nye Perram said in his judgment.

"In fact, however, this was only true if the consumer was careful to ensure that all of his or her off-peak allowance was exhausted before the peak allowance was fully utilised."