While it is true that the Telstra Copper lines are going to be cut off eventually, the fact is this won’t happen until 18 months after your site is declared ‘ready for NBN service’. Don’t be pressured by the big providers to hastily sign an over-priced phone plan.
You have 18 months from the time your site is declared ready, to arrange an NBN connection and to work out what you are going to do with your phone lines. It is highly recommended, however, not leaving this to the last minute. You should switch over well before the disconnection date to avoid the panic and to leave enough time for installation delays and porting your number. You can check the roll out status for your address on the NBN Co rollout map.