I was just wondering if I have to disclose faults with my car when trading it in for a new one. The issue is with my car

Customer: I was just wondering if I have to disclose faults with my car when trading it in for a new one. The issue is with my car hyundai they have rejected the warranty claim and have left me in the predicament of needing to purchase a new car as I’m afraid mine will break down. I rely on my car for work and need a reliable safe car for work and more importantly over the Xmas break.
JA: What state/territory are you in? It matters because laws vary by location.
Customer: Victoria I’m worried that if I disclose the fault with my car I will lose the $11000 trade in
JA: What steps have you taken so far?
Customer: i raised the issue about my i30 which is still under new car warranty that it was using excessive amounts of oil. They investigated it and found the fault that I require a new engine. They put the claim in to head office but they rejected it. The reason they did is that I supplied all the invoices and stamped service book to them but was missing three invoices for the 30000km, 45000km and 60000km. I don’t have these invoices because the business closed down and I have tried to get them as the mechanic who serviced it has his own business now and has continued to service it. He is unable to get the invoices for me as some business with a falling out with his business partner. My local Hyundai knows the business and explained this to head office but they have declined the warranty. I was told my car is twelve odd thousand to fix. Over the Xmas new year period I need a car but have been told it is still safe to drive but as long as I keep an eye on the oil level of which I do. I’m worried that whilst they appeal it as my mechanic is supplying a stat dec for the three services, that my car breaks down and I’m left with literally a car wreck. I’m in the predicament where I wait till mid January for Hyundai answer which I’ve been told is slim, drive my car and hope it doesn’t break down or go and purchase this new car this week and trade it in at the dealership and get the $11000 trade in for it. But I’m also concerned that the new owner who buys it is left with the issue and comes after me under section 54 of the ACL. The dealership “kia” went over my car to appraise it and drove it, never asked me how it was they said it drives like a dream and is well looked after. You wouldn’t know there is anything wrong with my car unless you check the oil between services. Drives normal, no engine lights are on and does not blow black smoke sorry big speal i find out today regarding finance with my bank so just checking
JA: Is there anything else the Lawyer should know before I connect you? Rest assured that they'll be able to help you.
Customer: i can drive this new car tomorrow if I can. All I want to know do I need to disclose the issue to the dealer when trading it in and if I don’t will the new owner once sold on. Can they come track me down and sue me? I just want to trade it in get my new car and live my life. To be honest I’m scared to drive my car over Xmas as if it does break down I’m basically left with scrap metal that is worthless. I’m in the predicament I’m unable to wait for this appeal with Hyundai as I don’t know how long it will take. Also I have emailed my local Hyundai to request all my warranty history on this issue in writing for my copy and also to get in writing that my car is still safe to drive as this was verbalised to me via the service department. I’ve followed everything to a tee with going through the claims process but after they rejected the claim I’m stuck between a rock and a hard place.