The short answer is 'it depends'. If you have FTTP, don't turn off the NTU. You can safely turn off your TPG supplied router, but I'd leave the NTU alone unless you're specifically advised to do something with it by TPG.
If you're on another ADSL-style NBN connection type, you can safely turn off your router while you're away, and your NBN service should work once your router starts back up when you get home (probably about 5 mins - 15 mins tops).
It really shouldn't matter to be honest, unless you're wanting to save a bit on power while you're away. Will it hurt anything to leave the devices on while you're away? Not unless a big storm rolls through really.