I'll weigh into this thread properly now.
Hertz NT have a $1100 excess on 4WD's, plus $4400 additional excess if it's a single vehicle accident. This is after applying all available removal of excess options from them.
If you have no reduction in liability, the amount is $4400 (plus $4400 if a single vehicle accident - a total of $8800!)
(They also have a piece of paper you must sign before renting that has further conditions including full liability for damage caused by collision with animals between dusk and dawn).
www.hertznt.com/terms/insurance.htm gives details about the liability (but not the additional conditions) and the website is down at the moment. I believe Thrifty for example has similar conditions.
What I'm trying to say is you really need to be sure your insurance will cover you for the amount you are required to pay in an accident situation. Excesses can add up to big $!