The announcement comes as Brown is linked to Triple Eight Race Engineering, to replace NASCAR-bound Shane van Gisbergen.
According to Erebus, the parting of ways is “an amicable agreement and early release from his contract”, after several years together, including in the Dunlop Super2 Series.
“Will has been a part of our team for almost six years and while we are disappointed to see him go, it will be business as usual for the remainder of 2023,” said Erebus CEO Barry Ryan.
“Our combined focus is on securing both titles and building Erebus to be the best team in pit lane.”
He said, “It's been an incredible journey with Erebus, and I'm grateful for the experiences and opportunities I've had with the team.
“This decision wasn't taken lightly, and I want to thank Erebus, the entire team, sponsors, and fans for their support during my time here.
“I'll be giving my all in the remaining races and remain 100 percent committed to our joint goal of winning the Drivers' and Teams' Championship with Erebus in 2023.”
Brown's current Erebus team-mate, Brodie Kostecki, will also make his Cup Series debut this weekend at The Brickyard, in a Richard Childress Racing entry.
He too is thought to be hold a contract with Erebus which theoretically ties him to the Betty Klimenko-owned squad for 2024.