Only a third of the drivers on the current grid hold contracts for Supercars' dawning of a new era in line with the new-for-2022 Gen3 regulation changes.
On Tuesday, it was confirmed Mostert will remain with WAU, which he joined last year, until at least the end of 2023.
Mostert signed a multi-year deal beginning in 2022, which will take him through to at least the end of the 2023 season.
WATCH: MOSTERT 'AT HOME' WITH WAU
It also ended rumours of a high-profile switch to rivals Red Bull Ampol Racing, with Jamie Whincup to retire from full-time competition at season’s end.
Mostert’s current WAU teammate Bryce Fullwood was also handed a contract extension late last year, with Jack Smith, Todd Hazelwood and Macauley Jones all locked in with Brad Jones Racing for 2021.
Mostert joins a number of drivers who have already confirmed their 2022 plans.
Shane van Gisbergen is currently tied to Triple Eight until the end of 2022.
WATCH: MOSTERT 'SUPER EXCITED' TO STAY WITH WAU
In February 2020, Nick Percat inked a two-year extension with BJR, just after David Reynolds signed with Kelly Grove Racing.
Prior to last year’s Bathurst 1000, Mark Winterbottom extended with Team 18 for two more years, with teammate Scott Pye currently off-contract.
The 2021 season marks the first full-time campaigns for Erebus duo Will Brown and Brodie Kostecki, the former signing a deal to race full-time with the team back in 2019. The team has affirmed both drivers will return next year.
That leaves Garry Jacobson, who confirmed prior to the season-opener that his agreement with Team Sydney runs to the end of 2022. His teammate Fabian Coulthard has only so far been confirmed for 2021.