Cheers Andrew,
The tyre pressures and trailer hook-up (ride height) were checked by the trailer hire guy. I guesstimated there was between 100-150kg of weight on the tow ball (it's recommended that you have 5-15% of the weight being towed on the tow ball), I stopped twice between Ararat and Ballarat to check that nothing had moved, and the car was tied down with webbing and rope plus the steel winch cable. I was cruising along at 90-95km/h and everything was handling fine until the long steep downhill section at Pikes Creek on the Western Hwy when the fishtailing started, and didn't stop despite attempted acceleration (the suggested method of correcting this problem).
Apparently, it is legal to tow up to 1.5 times the mass of the towing vehicle with a braked trailer - not that I'll ever try this again - the car will be delivered straight from the repairers to Racepace by truck!
Most importantly, AAMI (Ford) and Just Car Insurance (GTR) have covered the damage. Though I'm sure the insurance on the GTR will skyrocket next time the policy is up for renewal.