The unit you described is the Carin 520, and was offered by BMW as a factory option on Australian and European LTs. On my UK LT I added the VDO_Dayton MS5000 (Carin 522 equivalent) which is the same computer with a larger color display and a dedicated remote. I also moved this system to my US LT when I moved back to the States.
Read this thread
and this thread
for more info and pics, including links to get the updated mapping software. I'm not sure what maps are available now for Australia, but the links should get you started. You can also buy updated GPS operating software, though the manufacturer doesn't sell direct and I've had trouble finding a company that actually carries it, at least in the US.
Your unit has a smaller monochrome screen, and uses the radio buttons on the left handlebar to control it. There is a switch on the left of the smaller display that switches the buttons between GPS and Audio control. This uses a BMW-supplied wiring harness to splice in to the button wiring harness.
You can find a generic Carin wiring diagram here
. It's pretty poor quality, but it does give a basic idea of how the whole system goes together. Note that the diagram also includes several optional parts that don't come with every system.
I still have the original manual that came with my GPS that I would be glad to scan and send you if you like. I was able to use this GPS all over the UK, Europe, and America as long as I had the correct map CD loaded.
As far as keeping it, your GPS should work just fine if you have updated map CDs. The one on my old LT still works, though I don't ride that bike much anymore. But then again it also has the larger screen with much more map detail, rather than just a simple arrow and text directions.
If you're looking for a complete replacement unit, there are several great options available now. I'm using a Lowrance iWay 500c which has a great screen, lots of detail, and is fully waterproof (Lowrance started as a marine company). It uses NavTeq maps, so I'm sure there are Australian maps available. Try a search on this site or in Google for Lowrance iWay and see what comes up.
You can also poke around our GPS forum
for more info on bike suitable GPS units.
Feel free to ask any other questions that come up and we'll try to get you sorted out.