If you are under contract for an ADSL service, then they can still charge you the fees for breaking the contract up until the end of that contract. I'm not too sure about what happens when the ADSL is finally turned off, but that is usually 18 months after the NBN is switched on in an area. So your contract would have ended anyway.
The NBN is notorious for not turning up to appointments, so it's not too surprising. The communications between NBN and the RSPs is terrible and really needs to be addressed. They should be able to give Telstra, TPG,... notice that they aren't going to be able to attend on a particular day as soon as they know. NBNCo should also be given the contact number of the person requesting the service so that they can at least send them a text message with the details.