I'd add on a bit more for car hire - I'd be inclined to double it to be honest. Car hire is always more than you expect, there are often 'hidden extras' plus you have fuel on top of the rental costs. We recently holidayed in QLD and VIC (a week in each) and our car cost us $700 for each week plus fuel and that was with one of the cheaper rental places.

Have you double checked the price to get your car sent via the train too? Your estimate seems a bit cheap. I had my car sent over from Melbourne to Perth 11 years ago and it cost me $900.

If you were to drive, you may need to increase your allowance for fuel too. You haven't specified what sort of car you have but I know our Mazda CX7 costs almost $80 to fill in Perth where the prices are quite competitive. I've been over the Nullarbor and back and fuel is not cheap at all.

I hope that helps! Good luck organising it all