Have you tried to order VDSL? Chorus' map shows coverage there.
That crossed my mind, although it's possible that it's just dodgy internal wiring that'll be cleaned up with a full VDSL install (although I think I read that full installs aren't done by default anymore).
Those estimates are done using a mapping tool and a calculator.
Most of the time they are out by thousands or tens of thousands of dollars. Some times they are close but I would say about 70% of the time they are not even close from what I have seen.
I had one for 15k end up being 6k and another for 45k be 16k.
They can be submitted to Chorus for full costings.