"The court will make declarations which make it clear what the true position is, both in terms of square kilometres of geographic coverage and also the proportion of the Australian land mass that is covered by the respective networks," the judge wrote.
"Anything less than this may allow a misleading message or impression to remain."
Optus must also issue corrective advertising, the court ruled. Optus had argued it should not need to do this because the original ad was last aired more than 30 days ago on television, but the court disagreed.
"This judgment will be delivered less than 6 weeks after the advertisement ceased being broadcast ... By comparison to other cases in which corrective advertising has been ordered, the delay is relatively insignificant."
"Certainly, given the nature, extent and intensity of the advertising in question ... the length of time, of itself, does not suggest that the misapprehension created by the advertisement would have now lapsed."
The corrective advertising should appear in newspapers, websites of newspapers and displayed inside Optus shops at the point of sale and on Optus webpages, the court said.
In a statement, an Optus spokesman said the telco is currently considering its options.
"While we're disappointed in the court's ruling, Optus remains committed to the strength of our network."
"We have been consistent and transparent in how we communicate the less than 1 per cent difference in the population reach of the Optus mobile network compared to Telstra's, and these clear facts have not been in dispute."
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