It depends on the type of connection and the equipment you are using. Since NBN voice services are delivered via the internet and use an NBN box or modem/router which are mains powered, they will only work if they are connected to a back up power supply.
If you have a FTTP connection you can get a back up battery unit as part of the service which means that you may be able to continue to use the phone in the event of a power outage.
For other connection types, if you require a landline to be active at all time, for example if you are likely to need your voice services in an emergency you should consider a back up such as a mobile phone.
If this is important, when you sign up for a new service ask the provider what options are available to you.