Yes it will still work as it is backwards compatible. When Optus deploys their 4G network later this year, it will support the frequency and thus 4G would work on it. As Telstra uses 1800MHz, the new iPad will not support 4G over Telstra for the time being.
Optus had just announced that their 4G network will run on the same frequency as Telstra. This means that there will be no 4G in Australia that will run on the new iPad. Once our TV analog signals go off in 2013, that spectrum will be used for 4G. Optus just announced it this afternoon do it looks like there will be no 4G in Australia for another 3-4 years that will work on the new iPad 3. Hopefully iPhone 5 and iPad 4 will run on 1800MHz.