Good day @VictoraBerridge,
Welcome to TPG Community!
If it's an ADSL2+ modem and you'll be applying for our ADSL2+ unlimited data service, then it should work as long as the configuration on it is correct. (Ex: Correct TPG username and Password)
However, if the NBN is the available service for you and your modem is not NBN ready, then you might need to use the modem that we are providing. Also the home phone from the NBN service will only work with our modem, which means you'll give up your phone service once you use a different modem.