NextG is their 850 MHz 3G HSDPA network. Their 4G network is just branded "Telstra 4G" and "Telstra 4GX" for 700 MHz.

I thought it was kinda funny when telstra had "NextG" in their marketing, because now it's essentially "PreviousG"

Same with how the Hutchinson networks around the world (UK, Hong Kong, Sweden, etc) are branded "Three" even now that they have 4G networks running.