A site screaming out for a geo-mashup is the NSW Food Authority's Penalty Notices list, which shows the companies that have received penalty notices for poor hygiene and the like. Problem is, the data isn't in a very human-usable form, but would be very amenable to a mashup and map overlay!
I think it's time for Australia to get something like PlanningAlerts.com, which does this for local authority (council) planning lists. So, for example, it could fetch the list of development applications for my local council, and overlay them on a map. More importantly, you could register with the site and have it email you if there's a DA near your house.
The code for PlanningAlerts.com is open, but it's kinda UK-centric. Any ideas where to start doing mashups in Australia?