For free maps, check out the OSM maps that andrewp put together last year:
viewtopic.php?f=21&t=14234&start=150#p268841 Just copy the img file to your Garmin Montana and you should be able to enable it on the GPS.
I've not used the QLD or NSW maps, but the VIC and Tas maps are great.
For GPX trails, in Victoria, there are some publishers that have GPX trails for their publications, like OSP
http://osp.com.au/?cat=8 For other states, I usually look up various blogs of walkers who have done trails I am interested in. A few usually put up their GPX tracks, or will send them to you if you contact them.
But trails conditions can change, so it's best to not follow the trails blindly. If you haven't already, you can download Garmin's Basecamp app to preplan trips.
www.garmin.com/en-AU/shop/downloads/basecamp
If you download the OSM maps from above, you can have that display as the background map on Basecamp to make planning easier.