They have committed to having equivalent 4G coverage in its place, however, which will, for many reasons, provide a better alternative anyway.
If there wasn't a good enough reason to ensure your devices are 4GX compatible before, there is now. Make sure you ask your supplier if the devices support Telstra 4GX - that means all of Band 28. Band 28 is the all-important 700MHz frequency band which provides greater coverage and speed, and in many areas of Australia, the only Telstra 4G frequency band in use at all.
There are still PTT providers in Australia who sell 3G-only equipment, and even worse, supply 4G equipment which either does not support Band28, or only supports half of Band28 (Band 28A or 28B).
Any of this equipment will leave you caught out when the big switch happens. If you have a 3G-only device, it will become a souvenir. If you have a 4G device without Band 28, it will have very limited coverage. If you have a 4G device with half of Band 28, it will depend on which half your device supports as to the results. Currently Telstra uses one half and Optus uses the other, so you could be completely in the dark as if you didn't have Band 28 at all.
Use a reputable, experienced provider, trial equipment thoroughly before you buy, and ask questions to be sure.