Skybox has a nice new website and a team of typical tech starts up - young blokes and no women but not much more than press releases level information. I could find nothing on whether their satellites cover Australia and I suspect they probably don't. A UAV might be a more cost effective method of getting specific data targeted for the area you are interested in.
There is a strong rumour that in NSW LPI is working on a system for supplying cheap aerial imaging on the Clip and Snip model that they use for cadesteral and topographic data. This would certainly be cost competitive with Nearmap especially as many railway archaeologists don't necessarily need the most recent imagery.