Bushy
https://www.airservicesaustralia.com...erage_A100.jpg
As I understand it, the lilac shapes are coverage at 10000' - obviously lesser coverage below but it's a lot more than radar - and a lot more than where the airlines fly.
UTR
I believe the principle was response time more that detection. Although, I commend the ATC for a failure we had over water of comms and txpdr and they tracked us back on primary and checked the office to make sure we were home.
Extended surveillance offers extended flight following - possibly at a fee - that should provide benefit in those lilac squiggels.