I've done a hell of a lot of research since originally starting the post and have figured out how to make some great maps for Garmin devices. Field tested and works great.
Basically, there are a few ways to get better maps on the device. You have the OSM maps that others have compiled which are great due to the vector nature and you can get routable maps which I guess is sort of handy sometimes.
But if you want really detailed maps I've only come across 2 methods.
METHOD 1
1. Download or scan in you map source as JPEG. For example, for NSW maps you can get the digital topo maps for free at
https://maps.six.nsw.gov.au/etopo.html Then open pdf map in photoshop or editing program and crop out the map margins. Join multiple cropped maps in another photoshop file to cover a larger area. Save as JPEG. I found using the unsharp mask makes it much easier to read.
2. Overlay JPEG in google earth. You can use this
http://www.gelib.com/worldwide-utm-grid.htm to help align the map. Plenty of tutorials online to do this. Save overlayed image as KMZ.
3. That KMZ file won't work on the device because the Jpeg is too big. So you need to use the G-Raster software. Import the KMZ into G-Raster and save it for Garmin. Get it here
http://moagu.com/?page_id=155 It does cost but its cheap enough.
4. Copy the exported file to the custom maps folder of your Garmin device.
METHOD 2 (Much easier)
1. You need this software called MAPC2MAPC. You can download it as a trial it just overlays red crosses everywhere but you can try it out before you buy it.
http://www.the-thorns.org.uk/mapping/Still much cheaper than the Garmin Topo though. But this program is gold and support is awesome.
2. Use the 'work with online sources' > Edit the list of Map sources and here you can add the tile server of the map source.
For Example for NSW Six maps
https://maps.six.nsw.gov.au/arcgis/rest ... /MapServer you add it in as this:
NSW|https://maps.six.nsw.gov.au/arcgis/rest/services/public/NSW_Topo_Map/MapServer/tile/{z}/{y}/{x}
I don't understand what how you figure that out but send the developer the link of the tile server and he can help you out.
3. In the 'work with online map sources' you can define the area with coords. Use google earth to find the coords you need (NW & SE corners).
Then choose map source and Map zoom.
4. Once loaded you can optimise the map by sharpening and contrast. Experiment to how you like it.
Then you can save it as KMZ that works on Garmin devices without needing G-raster.
5. Copy the exported file to the custom maps folder of your Garmin device.
MAPC2MAPC also can load any jpeg map and calibrate it. Great software! The Developer also responds pretty quick support is great.
Few issues I'm still trying to figure out are:
1. Garmin 64s gives me a warning on startup that I've exceeded the tile limit. I've got 5 custom maps of a rather large size and only 3 show at a time. Kinda annoying.
2. Maps are a little on the laggy side when zooming in and out but honestly, it isn't going to hinder its functionality. The level of detail is awesome.
I found I still need the OSM maps as a general map but use the custom maps for the location I need in specific.
The Instructions are rather general but you should be able to fill in the blanks with a bit of googling. Let me know if I didn't explain anything enough. Hope this helps.
I'm hoping to experiment and learn a bit more about making maps but been too busy. Will post if I learn more.