Do yourself a favour and walk 5 meters either side of the Budget car rental counter at Darwin Airport to one of their competitors - the pricing won't be that different and there is quite a good chance of the customer service being better.
I had a small issue with my booking. Through arrogance the staff made this into a big issue and led to me cancelling my booking.
I would dread having to experience how Budget staff deal with actual problems that arise during the booking or on return of the car. I'm not a complainer regarding service, but know when I have been treated without the respect that I believe I deserve.