It may well be that investing in a Solar Power system in Australia will never be cheaper.
Why ?
1. The world market for solar panels is hugely over supplied and as such many manufacturers are selling even below marginal cost of production - Read this link
2. Whilst generous Australian Government rebates are still in effect, this may not last.
The Productivity Commission has called on the Government to scrap support for solar (read here) and the recent review of the Renewable Energy Target (which forms the basis of support for solar) suggested potential reductions support.