My understanding is:
If you're over the GMV, then you're unroadworthy. As GeeTee and Rod say, no insurance (including no CPT, so no public liability indemnity), and potential police/Dept of Transport hassles.
The question of "are you likely to be pulled over/get in trouble because of it" is an entirely different question. I would suggest the answer to that would depend largely on the mood of the person pulling you over at the time. I would also suggest that while there's probably a lot of other touring 4WDs loaded well over their GMV, said officer is probably not going to find that a very effective excuse. Let alone yourself should you need to live with having been involved in an accident.
As for the actual ball weight, I would be worried about 200kg. I would think that would be too much, and would make the car not behave properly. Even with airbags/good suspension/etc, that's still a lot of weight pushing down on the back. That's only one part of the equation though. Keep in mind in this situation you're also dealing with the GCM - Gross Combined Mass of the entire car+trailer package. That can work to your advantage.
IE, move things around. If your trailer is still within it's weight rating, but the car is over, are you able to move some heavy items from the car into the trailer (behind the axle if the download is too much), and move some light stuff forward? Also, rethink what you'll take. You mentioned the jerry's are contributing - do you *really* need to take that much fuel with you? That would be somewhere north of 120kg right there. Keep in mind there's generally quite accessible fuel up the cape, it's not like the Canning. It may be expensive, but so's a fine after being put on a weighbridge.
As for upgrading the GMV, my understanding is that there's 2 ways to do it:
* If you can do it pre-rego, then the build plate on the car gets replaced and that car effectively becomes a "one-off" with a higher GMV (and registered as such). When it's sold anywhere Aus-wide, it retains that rating.
* The other way is post-rego, where you can get a GMV upgrade performed and a mod-plate attached. But that's linked to the state where the mod was performed - IE, a mod plate from NSW might not allow the car to be legally sold in QLD. So it's more of a hassle when selling. Keep in mind that a GMV upgrade will probably need all new suspension and brakes rated for the higher GMV, even if you've got brand new stuff already. The stuff you have now is sized according to the max GMV that you currently are restricted to - you'd possibly need new stuff that conforms to the higher spec.
It all comes down to how happy you are in yourself with the risk involved. You'll probably be fine, the car is probably well over-engineered to take this sort of thing into account, and it probably won't be an issue. As mentioned, thousands of others get around with well overweight cars with no problems. BUT, if you DID have an issue, even a minor one, is it worth it to you?
Finally, good work on putting it over the weighbridge pre-emptively. A lot of people wouldn't even think of it, or would prefer not to know. Well done for taking your family's safety (and the safety of the other road users) into your own hands and actually figuring this stuff out. Sure, it's a hassle to do it now so close to the trip, but once done you won't need to worry about it.
Good luck!
Matto
