The ruling opens the way for Optus to seek damages - depending on the extent to which it can establish Telstra's breach of the agreement that governed its access to Telstra's wholesale network.
Optus had alleged that in the 1990s its confidential traffic information was being passed between Telstra Wholesale and Telstra Retail.
The information related to the number of Optus long distance calls that originated and terminated on Telstra's network.
At the time, Optus managed only the long distance trunking aspect of its customer's calls. The calls both started and finished up on Telstra infrastructure.
The required interconnection between the networks to make this possible made certain information about each call available, including quantity, source, destination, duration, time of occurrence and type of telecommunications traffic.
It was alleged that Telstra Wholesale passed some of this data to Telstra Retail, who used the information to prepare ‘market share' reports.
Optus argued these reports formed the basis of advertising and marketing attacks in the long distance call market "to lure Optus customers back to Telstra and gain market share".
Access to the information also occured without Optus' knowledge or consent, the number two telco said.
"The sharing of information between Telstra Wholesale and Retail meant that, in real time, Telstra Retail effectively tracked the success of Optus marketing initiatives and special offers and was able to monitor market share performance for marketing purposes," Optus said.
Telstra denied the information it used in the reports breached its access agreement with Optus but Justice Edmonds found otherwise.
"This is another example of anti-competitive behaviour from Telstra," said Maha Krishnipillai, Optus' director of government and corporate affairs.
"It is more historical evidence of why Telstra should be separated as canvassed in the Government's regulatory reform discussion white paper [released at the time of the NBN announcement]."
Telstra declined to comment to iTnews.