Telstra has officially taken the competition watchdog's side in its legal battle over regional mobile network roaming with Vodafone.
Justice Griffiths in the federal court today allowed Telstra to join the ACCC as a respondent to Vodafone's action against the regulator.
Vodafone launched its court battle earlier this month after the ACCC issued its draft decision on whether it should declare a wholesale domestic mobile roaming service.
It said there was not enough evidence to show that forcing telcos to allow rivals to roam onto their regional mobile networks would improve competition for regional users.
Vodafone lashed out at the ruling, claiming the ACCC had used a "flawed" process to arrive at its decision, which Vodafone said left regional Australia "hostage to Telstra".
It took the ACCC to court to force a judicial review of the draft decision ahead of the ACCC's formal ruling, expected late this year.
Telstra - which has the country's largest regional mobile network - would be the most affected should the ACCC decide to go ahead with the proposal.
For this reason the telco asked the court that it be allowed to take the ACCC's side in the proceedings.
Justice Griffiths agreed it was undeniable that Telstra had a strong interest in the outcome and granted the request.
He also allowed Optus to join as an intervener in the case, which gives it the ability to submit evidence and cross-examine without being aligned to any one side.
The matter is set down for a further case management hearing on September 4, and is scheduled for a full hearing over September 27 and 28.
Vodafone's argument centres on how the ACCC arrived at its decision: the telco says the regulator conducted the inquiry without issuing a detailed description of what a domestic mobile roaming service would look like, but still asked for final submissions on the matter from interested parties.
The telco claims it is impossible to properly work out whether a domestic mobile roaming service is in the long term interest of end users without such a description, which would contain things like any limitations or restrictions.