Optus has won the right to seek a percentage of Telstra's profits in damages after the Federal Court found Telstra breached confidentiality when it used Optus' call information to prepare market share reports.
The ruling was made by the full bench of the Federal Court yesterday.
Optus alleged the reports "formed the basis of marketing and advertising attacks in the long distance call market to lure Optus customers back to Telstra and gain market share".
It was unclear if Telstra would appeal the judgment.
A Telstra spokesman told iTnews: "We are still reviewing the judge's decision and have yet to make a decision as to whether to appeal or not."
Optus director of government and corporate affairs Maha Krishnapillai labelled the ruling a "significant victory".
"Optus will now move to quantify our entitlement to contractual damages (which we expect to be significant) and also seek to pursue Telstra for an ‘account of profits' that were gained by their use of our confidential traffic information," Krishnapillai said.
Optus lodged an appeal last month after previous judgments did not rule on the issue of whether or not confidence was breached.
Yesterday's outcome meant Telstra breached its access agreement contract with Optus and misused the telco's confidential traffic information.
Telstra had disputed whether the data was confidential. It argued it owned the information because, in the 1990s, "Optus' customers were Telstra's."
Optus was already pursuing damages over the contract breach.
Another reason for separation
Krishnapillai said the case was a "good example of how Telstra has a very real ability to act unfairly under the present regulatory system."
"This result shows that without full, transparent and independently assessed structural separation Telstra will have an ongoing opportunity to seek to significantly damage competition to benefit its own agenda," Krishnapillai said.
"It is disappointing that Telstra was prepared to misuse confidential information in an effort to stifle competition," he said.
"It's [also] unfortunate that Optus has had to spend many years in court getting justice when this all could have been avoided if the regulatory separation had happened many years ago."