Will my warranty be void if my car isn't serviced at the dealership?

NO, PROVIDING he follows the Holden service schedule and does everything by the book. If you have a problem in the future that you want to have fixed by Holden under the warranty, you need to be able to prove it has been serviced correctly, so there can be no suggestion the problem has been caused by the non-factory servicing. Make sure your mechanic is doing the work by the book and everything is being recorded.