Wow your current electricity must be really expensive... or your solar setup is?? Also the calculation was done by the company selling it, I'm pretty sure that was far too optimistic.
With a calculation like that I'd buy solar in a heart beat!
But as I said, my main concern is my winter bill, which I know by asking people who own solar in the region, is almost the same with solar since there's no sun at all (it's not that it's cloudy, it's that the entire day duration is like 4 hours - under which you barely feel any sun heat and in practice the solar output is exactly zero on the worst month and near zero for 4 or so months). Hence the very long ROI here, but I agree that for Queensland, 10 years would be a bit too long if you dimension the solar array properly.
You are mere hundreds of miles from cheap Chinese panel manufacturing. The Chinese are going to pump manufacturing oversupply for likely a decade more.