there are bitcoin exchanges that accept AUD (MtGox, btcmarkets, LocalBitcoins)
there are several OTC bitcoin outfits with offices in Australia
a number of exchanges allow deposits to be made in australian banks (CBA, Westpac, etc)
how 'liquid' BTC is depends on where you're trading BTC, and is subject to change. the size of the exchange is obviously the biggest factor; most people are satisfied with the liquidity exchanges like Bitstamp and MtGox provide, but the volumes traded (as an absolute quantity or proportion of total volume of bitcoins) are nowhere what proper financial markets experience. MtGox is facing serious legal challenges so might be very liquid in the morning and fucked in the afternoon.
if you're not trading in huge quantities you will probably be satisfied, and subject to very little risk that you won't be able to cash out.
reporting bitcoin earnings to the ATO is currently a nightmare.