Brown, who was under contract with Erebus until the end of the 2024 Supercars season, recently emerged as the front-runner to replace NASCAR-bound van Gisbergen.
It was thought that Erebus majority owner Betty Klimenko had informally granted a release.
That release has now been formalised with Brown's impending exit now confirmed by Erebus.
“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,” Erebus CEO Barry Ryan said.
“Our combined focus is on securing both titles and building Erebus to be the best team in pit lane.”
As flagged by Ryan, Brown has been aligned with Erebus since early in his junior racing career.
In 2019 the team committed to running him in a main game Supercars entry in 2021, despite still having two drivers – David Reynolds and Anton de Pasquale – under contract.
Both ended up leaving the team at the end of 2020, paving the way for Brown to step up alongside Brodie Kostecki.
Brown has this year emerged as a title contender thanks to Erebus' fast start to the Gen3 era.
He currently sits second in the points, 41 behind Kostecki and just ahead of Triple Eight drivers van Gisbergen and Broc Feeney.
Erebus is also locked in a heated battle with T8 in the teams' championship, the rivalry turning bitter at times, including run-ins in the stewards rooms in Perth and Sydney.
Speaking about the split with the team, Brown promised to keep pushing for both titles this year ahead of his exit.
"It’s been an incredible journey with Erebus, and I’m grateful for the experiences and opportunities I’ve had with the team,” Brown said.
“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 per cent committed to our joint goal of winning the driver’s and team’s championship with Erebus in 2023.”
