Finally I have had some clear information from my case manager. The delay has occurred because Telstra sat on my ADSL order, doing absolutely nothing with it, for more than the 10 day window that the churn process allows. They lost their chance so now have to go through an escalated procedure with my old provider, which requires giving the latter 5 working days notice.
If (a big if!) all goes according to current plan, I will have a connection on Friday. It will have taken Telstra 4 weeks to simply churn an existing, connected, functional ADSL connection. This has happened because it has just allowed my order to languish, despite my spending many hours on the phone to them trying to get something happening. Telstra, in my experience, is an utterly worthless company. It has treated me (supposedly a paying customer) with complete contempt. I return the favour.