I bought this devise in USA last year where it worked very well.I fished it out the other day but it doesn't work here why?

Each Garmin is only provided with one set of maps. Your device probably only has North America maps. To check go here:
Tools->settings->Map->map info (at the bottom of the screen)

This screen will list what maps you currently have.
AUS maps can be bought here for just under 200 AUD
City Navigator Australia New Zealand NT Garmin

You may find free maps here but they are complicated and user created maps not professionally done like Garmin maps.
OpenStreetMap

When you mean does not work, does the unit power up or is it plain dead? If it does not turn on, then you will need to troubleshoot or send out for repair.

If it turns on, but does not provide you with maps, you need to load maps for Australia as you currently have US maps loaded on the device.

Google search for Australia maps for your unit, or contact Garmin customer service.