Can anyone advise how we could go about getting such a database? Would it be a CASA responsibility (surely they are interested in air safety) and how could that be organised?
CASA don't manage data, and I don't expect that they have any resources to manage and maintain such a database. In this instance I don't think it's appropriate to get stuck into them about it. (Don't get me started on comnunity service flights though...) It's probably easy enough to pull the powerline infrastructure data from the respective network providers, (especially if CASA broker the request) but you probably will miss a lot of data where power poles are on private property, eg. the SWER (single line earth return) type arrangements which are typically the biggest hazard.
I wonder if the Avplan/Ozrunway guys have looked at this? If there is any usable data out there it would not be hard to add another layer, and add an alerting function.