Hi David,
It looks as though your not quite understanding what currency is used to trade.
If you are to invest in any platform on US stocks (i.e IB or spectrum live), what happens, is when you make a stock or option trade (eg. long, short or option margin position) it is taken as US currency.
What your probably referring to is your "base currency", which if Aus is chosen, a loan (or forex position) is taken to make the US trade (even if AUS is your base currency). The problem here is that if it were a long term position, and the US currency went down, you will actually loose money on the position even if you sell the stock for what you brought it at.
Having said this, you cant do anything about it unless you hedge your currency position (which can be done with IB), however the likely loss (or gain) due to currency exposure is minimal.
I am australian, i live in the UK, and trade on US stocks. For that reason i leave my base currency in GBP (confusing i know). If you live in Australia, spend money in australia, have debt in australia and have a AUS$ bank account, i would DEFINATLEY leave your base currency in AUS.
Hope that helps.
Matt