Exchange traded commodities might be the way to go. I don't think there is a short gold ETC on the ASX (there is SBUL listed in London), but you might be able to take a short position in a long ETC via your broker. Have a look on that link for more information.
Futures or CFDs might be the easier way to go. I'm not an expert though - I'm sure someone with first hand experience in it will comment soon.
If you look under the ASX code GOLD you will find a company that's SP reflects movements in the price of gold, you could short that if you wanted to...
You can also buy a put option on gold or short a Gold future. Or you can also buy a put warrant on Gold. Of course, you can short some ETF for gold. The biggest ETF for gold I know is SPDR, they are holding 1114.44 tones of GOLD.