Hi Kane, essentially the conduct of the ute seller is misleading and deceptive conduct (fasle advertising by ommission), which can be legally actioned. Essentially, it is based on the premise that you thought you were getting a particular deal when in fact you were not.
The cheapest and easiest way to action this is to go through state government bodies, otherwise, you will be going backwards on the cost of a solicitor in the short term until the matter is finalised. The best place to start is http://www.fairtrading.nsw.gov.au/ftw/About_us.page?
You can also get in touch with your local community legal centre, who are free. They are very experienced solicitors, who work in firms and donate their time on a pro bono basis to the local legal centres. One that I can recommend is http://www.klc.unsw.edu.au/ . They are very good and attached to a group of 8 university and are arguably one of Australia's best law schools - http://www.go8.edu.au/home
No matter who you get of the above two to action the matter for you. The first step remains the same, namely, the will draft a "letter of demand" and serve it on the ute seller.
This is not legal advice, as I am primarily a corporate and commercial lawyer. It would be uneconomical for me to take on such matters, even if I were permitted by my firm.
Best of luck with the matter.