There seems to be a widespread belief that you shouldn't upgrade your business telephone system before NBN arrives because you will have to upgrade it again once NBN is available. This is quite false and seems to be being spread by IT people whose core business is not telephony. If you need to upgrade your phone system because the old one is too small, doesn't offer the features you now need or is failing then go ahead and upgrade rather than struggle. In some cases action is required now while NBN may not be available for another 2 years.
If you don't want to move to a SIP trunk just yet, the new system can be configured to take your existing analogue or digital lines and can be licensed up or equipped to be ready when you do want to make the change so the changeover cost will be minimal.
Be aware that ISDN from Telstra is now End Of Sale and therefore it is not possible to get additional lines or new lines if you move premises. Other carriers may still offer them but will probably follow Telstra's lead and no longer offer them. If you are getting a new system then there is really no reason not to change to a SIP trunk as you will have to do it sooner rather than later anyway. By changing at the same time as getting a new phone system you can enjoy the improved flexibility, number portability and reduced cost. It may enable you to eliminate GSM gateways too and allow proper control of your outgoing caller ID so people can contact your staff back quickly and easily.
Also be aware that many newer systems are natively compatible and may only require a few hundred dollars worth of hardware or licensing to accomodate a SIP trunk rather than complete replacement.