I think it would be helpful to give some background.
Some years ago, various Australian state governments (there are 6 Australian states) offered substantial incentives to homeowners to install solar panels on their rooftops. The scheme varied from state to state, but generally it was along the lines that, there were subsidies for the instal, and a contract to sell excess electricity back to the grid at the same rate electricity companies sell power.
The scheme was far more successful than expected, to the point it became a problem. State governments responded in stages by first reducing then eliminating subsidies, and the buy back rate of electricity was reduced substantially.
There is no doubt that those who got in early got a good deal. Those deals are long gone. An offsetting factor is that the cost of such systems has fallen dramatically over time. The 18 panel system referred to by the OP is probably a 3.6kW system. I have recently received an ad from the people who installed mine, for a 6kW system for only A$3000. (I got a reasonable deal on a small system, but nothing like the deal some of the first people got.)
I suspect that the OP was caught in an intermediate stage where state govts were trying to reduce the subsidies. A few years ago, my state govt tried to do something underhand in that regard, but did not get away with it.
For me, a far bigger issue in Australia is why over a 20 year period, Australia went from having the cheapest electricity in the Western world to the most expensive. Some people blame privatization, but look at WA where the entire system is still 100% govt owned. The reason is complicated, but at its core is a major government failure.