Welcome to the community!
What type of NBN connection you have with TPG? If it is a TPG-FTTN it will require the old Copper lines used from ADSL2+ service to operate and the old service from the previous ISP will be disconnected.
In the event that your new NBN service with TPG has been installed and now working, you have the option to keep your old service until the end of the billing cycle. But just to set your expectation TPG do not have control with your old service and you are responsible to cancel it.
Let me know should you require further assistance.