The NSW State of Origin star had been in talks to sign with the Melbourne Storm over recent weeks after he was granted a release from Parramatta at the end of last season to pursue an R360 contract.
Parramatta claim that as part of agreeing to release Lomax from his contract, they would not let the goalkicking outside back join a rival NRL club without their permission until 2028.
On Thursday the Eels released a statement saying mediation talks with Lomax and the Storm had broken down, prompting them to take the 26-year-old to court.
"It is disappointing that we have reached this position, but we have an obligation to the club's stakeholders to protect the contractual rights of our club," Eels chairman Matthew Beach said in a statement.
Lomax looked all set to join the breakaway rugby union competition, but when R360's kick-off date was pushed back from 2026 to 2028, he was left in limbo.
The former St George Illawarra star then became an obvious target for Melbourne, who have farewelled Ryan Papenhuyzen and Nelson Asofa-Solomona since their grand final loss to Brisbane.
The Eels, however, have stood their ground over the terms of Lomax's release and have said they would only waive their agreement with the winger if they were properly compensated by Melbourne.
There has been widespread speculation the Storm would release players in order to get Lomax out of his contractual bind with the Eels.
But finding middle ground has been hard.
Given their off-season turnover the Storm are unwilling to let go of any of their top names to get their hands on Lomax.
And given Lomax's standing as one of the game's premier outside backs over recent seasons, the Eels would likely feel short changed by any proposed trade.
The players' union the RLPA has also previously warned against clubs using players as pawns in negotiations with NRL rivals.
"The guiding position of our club has been to ensure a fair exchange of value for our football program in circumstances where the Storm are attempting to obtain the benefit," Beach said.
"Zac had legal representation during the negotiations of his release.
"Zac accepted those conditions on the basis that he told us that his interests were focused on pursuing opportunities with rugby union, particularly R360.
"The release documentation was registered with the NRL. The NRL are aware of the conditions associated with the release.
"Our club believes in the importance of observing contractual obligations."
Lomax, a three-cap Australian international, only joined the Eels for the 2025 season after seeking a release from the final two seasons of his contract with the Dragons.
His exit from the Red V appeared to stem from his dissatisfaction with being moved from the centres to the wing by St George Illawarra coach Shane Flanagan.
In 19 games across his single season at Parramatta, Lomax scored nine tries and kicked 53 goals.
The Eels have appointed Arthur Moses to lead their case, which is listed for Friday at the NSW Supreme Court.
