A currency depends on export-import trade balance. A high export country, and a net exporter will push up the currency as demand for the goods outpace the exchange of other currency for importation. In this case Australia sells iron ores, gold, and other minerals and form the export basis and the value is much higher than imports. When commodity prices are at all time high, the AUD$ also reach all time high due to demand for AUD$ to purchase those commodities.
Now the commodity prices has slumped and trade balance is approaching negative. The AUD$ demand will be slowing down, as less amount of export compared to imports. To revitalize the trade balance the currency is deemed overvalue (above fair value), and needs to be reduced to boost/balance export. The AUD$ is dropping slowly is due to the high interest rate in a world where there is no more high interest rates. Demand for the high interest rates is what keeping the currency in somewhat high demand. The government wants the currency to become lower as the
large amount of AUD$ in FD is not going to help the economy.Lets say price of commodity A price unchanged at USD $1 per kg. When AUD$ was 1:1 with USD, Australia company will get 1 AUD$ for every kg sold. Now lets say the exchange rate is AUD$ 0.9:1 USD$, The same company will get 1.11 AUD$ per kg. Also with cheaper exchange rate, it will balance the price dropping in terms of USD$, and the company earning will still be same. A cheaper AUD$ will also reduce import as the inverse happens, it is costlier to import.
To reduce the value of the AUD$, the primary function is via interest rate control, lower the interest rate the currency will drop, raise it will rise. Bust it also depend on other macro economics such as trade balance, economic growth, sovereign bond ratings & foreign currency reserves.
Lowering of currency = boost export, reduce import and improve the economy but raise inflation.

Many many thanks Professor Gark!!! You should write a few books on economy, investment and also not forgetting Palm Oil close to your heart!
The recent drop in Aust's commodity prices explains the drop in A$ (spill over effect from China's slow down). A$ - US$ rate is not a measure of whether it is overvalued or not.
Got it now.