Mostert has had an association with BMW and its racing program dating back to 2017 when he drove for the factory-supported AAI Motorsports in a BMW M6 GT3 at the Macau Grand Prix.
He later went on to race for the German marque in the Bathurst 12 Hour under the BMW Team Schnitzer banner in 2018 and 2019.
However, speaking on the Loud Pedal Podcast, Mostert said he and BMW have parted company recently.
The 28-year-old cited the global coronavirus pandemic and travel restrictions for the split.
“Probably a lot of people don't know, during this COVID period I decided to freelance and split with BMW for the time being,” said Mostert.
“Just being locked down here in Australia as well, obviously there's no opportunities to race a GT car at the moment.
“I will always have that connection with those guys and we'd love to something in the future with them. (It was) 100 percent a mutual decision.”
Mostert remains optimistic about his future in GT racing and said a return with BMW wasn't out of the question.
“Like I said, hopefully there's an opportunity to go back and race with those guys,” said Mostert.
“But (I) just didn't see the point of being tied down at the moment during (a period) where you just aren't able to go overseas.
“We'll see what happens in the future. I'd love to drive some different type of GT cars and see what all the buzz is about that, but yeah, we'll just see what happens in the future.”
The Robson Civil Project Townsville SuperSprint takes place on September 5-6.