The 21-time race winner will continue into a fifth year with WAU next season and then beyond thanks to an extension of his deal.
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“I'm pumped to be continuing on here at WAU – this team is like family, so I couldn't picture being anywhere else,” said Mostert.
“It wasn't a hard decision, that's for sure, it all came together pretty quickly. Everyone from our team to our partners and all the WAU fans have been so amazing, so I'm really excited to continue that journey together.
“We came here with a few big goals in mind, and we haven't achieved all of them just yet, so I'm as motivated as day one here to do that.
“A huge thanks to everyone involved in the team and our partners for the faith they put in me, representing them is really an honour so I can't wait to see what we can continue to achieve together in the years to come.”
WAU team boss Bruce Stewart said extending Mostert's deal was a “no-brainer”.
“We are delighted to continue our journey with Chaz long into the future together,” he said.
“He has been a great fit for our team both on and off-track, so it was a no-brainer really.
“Ever since day one he's played an amazing role in our team, what you see is what you get with Chaz.
“We've shared some incredible moments together so far, but we are certainly not any less focussed or driven when we first started together.
“We've got a big weekend here on the Gold Coast and then Adelaide to finish 2023 in the best possible way, but I'm also really looking forward to seeing the role Chaz plays in helping guide and mentor Ryan Wood through his rookie year.”