Telstra and NBN Co will square off in the NSW Supreme Court in November over the price NBN Co should pay for access to Telstra infrastructure.
The court case centres on a 2011, $9 billion infrastructure services contract for NBN Co to lease Telstra infrastructure such as pits, pipes, ducts and exchanges.
The two telecommunications giants dispute when the consumer price index for payments covered by this agreement should kick in.
The CPI applies every year from January, and Telstra believes the CPI for payments under the agreement should be effective from 1 January 2012, given the signing of the NBN Co contract in 2011.
NBN Co will argue that the CPI should apply the following year, from 1 January 2013, given the ACCC's approval of the structural separation undertaking (SSU) and the commencement of that undertaking in March 2012.
Telstra and NBN Co have been in mediation over the issue for the past year but have not been able to come to a resolution.
"We have not been able to reach agreement through a long mediation process so, as provided for in the contract and as the next step, Telstra is asking the Court to decide," a Telstra spokesperson said.
"The impact to us over the term of the agreement is significant, but not material from a market perspective."
Telstra declined to provide the dollar value of the difference in CPI, but it is understood to be worth over $100 million.
A spokesperson said the case would have no impact on the renegotation of the NBN deal but it would give Telstra "clarity" on a disagreement the pair had had for a long time.
The first directions hearings for the case is set for November 8, 2013.
NBN Co declined to comment.