in Australia , the insurance is more expensive than the net rental price. the price of insurance purchase from car rental is a total rip off,if you don't buy it, the so called cdw deductible is at least aud10000, are you sure you are comfortable with it? and because you don't purchased through them, they may tend to be more picky on examining your car when you return it.
the usual travel insurance that you can buy in HK would normally offer rental excess between hkd 3000 to 5000.
other options are selecting not those major car rentals, such as apex, ace, juice etc, their insurance is more reasonable. but their locations are off sites.