In this case the reason for the delay is that have had massive delays in getting equipment which affects when installations can be done (see here). They have known about the problem for weeks and they should have informed you, but their communications have been terrible.
The body that handles consumer complaints about Phone/ISP's is the Telecommunications Industry Ombudsman (TIO).
As for new cable (phone or HFC, depending on your connection type), that is entirely up to the NBN.
Toll Logistics suffered a cyber attack in late January.
Toll handle the Transport logistics, warehousing and deliveries for several companies including Optus and are only getting back up to speed in the last few days, so it may take a while for the backlog of modems, phones, sim cards etc to be delivered.
If you were an NBN Belong Customer and moving to Optus NBN they would not be running a new cable.
If you were using a different technology for home phone and internet and the area has gone live on the NBN, Optus maybe waiting for an NBN technician to be available to complete the connection.
I and other people I know who have changed Telco providers have kept the previous service going until the new one is running and then cancelled old service where applicable.