It's far too difficult to answer such a broad question like why the AUD is so undervalued without going into great depth, but I'll try to give a brief overview.
According to The Economist's "Big Mac Index" (which works on the principle of purchasing power parity) the fair value of the AUD is US$0.86 (assuming a Big Mac costs A$3 and US$2.59). Also, the average of the AUD from 1986-2000 was US$0.73.
Both of these point to a very large undervaluation of the AUD compared to the USD. However, when compared to all other currencies, excluding the yen, the AUD has remained faily stable since the early 1990's. So the quick answer is that the AUD is low because the USD is high (which it is), which occured during the late 90's when the world was "in love with all things American" during which the USA experienced a triple boom in its economic growth, it's sharemarkets and its currency.
It should be considered that from a historical perspective, Australia's current economic condition (superior economic growth, a strong terms of trade, falling unemployment and a good control of limits to sustainable growth - inflation and trade balance/CAD) would normally have meant that the AUD would be moving up strongly.
Also, keep in mind that a low AUD is not necessarily a bad thing. It makes Australian goods more competitive against foreign goods and this leads to stronger economic growth. And due to microeconomic reforms, in particular labour market deregulation and the tarrif phase-down, have put pressures on firms to absorb increases in costs rather than pass them on, thus keeping inflation low.
I hope that helps!