Telstra has succeeded in getting a court injunction to stop Optus running ads claiming it operates the best mobile network in Australia.
Victoria's Supreme Court granted the injunction on Wednesday, saying that SingTel-owned Optus should "take all necessary steps to withdraw any advertisements or promotional material containing the alleged representation".
"I am satisfied that there is a serious question to be tried in relation to the allegations of misleading or deceptive conduct on the part of Optus," Justice Ross Robson said.
Telstra had argued the ads should be removed from billboards in Queensland, NSW and Victoria as well as websites because Optus's claim it is "undisputedly operating the best national mobile network" are misleading and ran the risk of convincing ordinary Australians to sign up with Optus because Telstra no longer has the best mobile network.
But Optus had said the slogan was a wry reference to history and technological change and described Telstra's analysis as an "artful construct" without reason.
The ads stated: "Empires end. That's what they do", and that "The Optus Mobile Network has been ranked the best overall in voice and data", adding in fine print that this was backed by the P3 Mobile Benchmark report released in December 2017.
Justice Robson said while consumers know it is common for companies to "big-note themselves" in ads, what made this one potentially misleading or deceptive was its reference to the P3 report.
The report had found that Optus overtook Telstra "by just three points" in a "narrow margin."
"The content of the 2017 P3 report does not support the proposition that there has been a significant and permanent change in the relative market positions of Telstra and Optus, nor does it support the idea that Optus is now undisputedly operating a better mobile network overall than Telstra," Justice Robson said.
The judge said Telstra may suffer "irreparable injury for which damages would not adequately compensate" if the ads were not removed.
"If the injunction is not granted, but then at trial Optus is found to have engaged in misleading or deceptive conduct causing Telstra loss, then Telstra will have lost some customers and suffered some injury to its brand - both of which will be difficult to quantify and to compensate adequately," he said.
He is yet to rule on the issues of legal costs.