I already told Optus I can't be at home for my NBN appointment, then I received a message from NBN saying they are coming on Thursday anyway. I spoke to Optus and they said they can not change the date and the only way is to wait until Thursday, tell the tech I am not at home, then they will schedule another date. It seems very strange! And they said there will be no cost, but I am concerned that there will be in the end.
Why can't I just let the tech know now I won't be at home instead of waiting for them to call on the day?
Optus and NBN are two differnt companies. So while they should act in concert there can be communication difficulties. That said, it is Optus sole job to communicate with the NBN. You have done your bit and communicated with Optus so whether they tell the NBN is up to them. You are an Optus customer, Optus is an NBN customer.
In practice if you are not home then the NBN will arrive and leave and that's it. At this stage all you need to do is book another NBN date. You can do this now.