I'm a complete newbie and I am not sure where you want maps for. Here are listed free Garmin topo maps for Australia. I also have one called shonkymaps. Maybe not what you want but there are links about to other topo maps and resources.
I found this site by googling it, so follow this link as it gives free topographical maps for Garmin units at 5 metre intervals. I haven't installed one but have considered it.