Depends if you buy the unit in Australia (generally expensive) or from the US. Either way the Garmin maps of Aus are rubbish so you'll need to load your own maps. As far as I know there aren't any toppos for Australia from Garmin but thats okay because there are other options.
I use the free "Shonkymaps" software that you can download from the GPS Australia forum web site (you just have to join (for free) first). I also use "Contours Australia" which is an overlay that has much closer contour intervals than Shonky and is also free on that site. I believe you can also buy another product that is pretty similar to Shonky - but from what I've read it isn't worth it considering the price and slight improvements over Shonky.
I think when you get the unit it will be using the degrees/minitues/seconds format but it can be easily configured to spit out pretty much any type of location format. You can set it to give grid co-ordinates that work with the Tasmaps no problems at all. Just note that the different series' of Tasmaps use different datums so if you use 1:25000 as well as the national parks maps for example, you'll have to change the datum in the GPS so that the grid reference it spits out suits the particular map.
oh - just realised you are a New South Walian so you probably couldn't give a toss about Tasmaps! Maybe when you come and visit