The Supercars series leader will drive a latest-spec Audi R8 in the GTWCA opener, partnering Liam Talbot in place of Talbot's regular co-driver Fraser Ross.
The one-off deal for Mostert follows a part-time GT programme last season in an Audi sharing with Tony Bates.
"I didn't plan on doing anything other than Supercars this year, but I'm definitely happy to help Liam out for the Phillip Island,” said Mostert.
"Liam and I actually have a mutual friend Ben Hennock from Wash It Australia. He supports both of us, and my name was thrown into the ring and I was keen to help them out when Fraser got caught up and was unavailable.
"I haven't driven the new EVO 2022 package, and I can only rock up just before qualifying, so I’ll be jumping into the deep end, so I’ll try and get into the swing of it as quickly as I can.
"Like any race car driver, we are there to compete and we want to be on the top step. I have some experience with the Audi from last year, but Liam is very capable driver and we’ll be hoping to put his car as far up the order as we can.
“The competition in GT is very stiff. There’s lots of good cars, teams and drivers there. It should be a great weekend.”
Talbot heads into the Island opener in good form, he and Ross having won the final race of the 2021 season at Bathurst late last year.
“It’s all a last minute thing, but it will be good to be in the car, and will be nice to learn a few things off Chaz. Hopefully we are in the fight for race wins,” said Talbot.
“From what I understand, the EVO 2022 is more stable in the rear at high speed. Phillip Island is an aero track, so to have that stability and confidence in the rear at a circuit like this will be much appreciated.
“Naturally, you want to get the best result possible. Having not done a test in the EVO, it will interesting on how quickly we come to grips with it, but it will be nice to get some poles and wins.
“I’m just really happy to be part of the competition and excited to go back racing.”
Mostert isn't the only Supercars driver on the GT grid this weekend, with David Reynolds in another Audi and Shane van Gisbergen sharing a Mercedes with Prince Jefri Ibrahim.
Audi factory man Chris Mies will also be there to partner reigning Aussie GT champion Yasser Shahin.
The GT World Challenge Australia action from Phillip Island will be shown live and free on Motorsport.tv (local restrictions apply).
