There is a topic out there that covers this.
See
http://bushwalk.com/forum/viewtopic.php?f=21&t=415The First post references the Telstra coverage maps:
http://www.telstra.com.au/mobile-phones/coverage-networks/our-coverage/state-coverage/Disclaimer: I am a Telstra engineer - in SA.
Basic idea is that the Next G antennas are somewhere out towards Hobart - probably above Maydena and a few other highpoints elsewhere.
As long as there is line-of-sight, coverage should be out to about 60km range. But performance decreases with distance - hence the different shadings in the coverage maps.
As for my experience - I probably mentioned getting coverage on the Anne saddle before it drops down to Shelf Camp - and on the Anne NE Ridge
But we also had coverage on Hesperus, Sirius, Pegasus and even low down in the saddle between Orion and Pegasus.
I would expect to be able to get coverage on the ridgetops anywhere between Hesperus and Scorpio - places with a clear view towards Picton.
Having said that, coverage does get affected by congestion [too many using it at the same time], some atmospherics [heavy precipitation is bad] and the quality of the handset will also come into play.
Telstra market the phones with better range as having a Blue Tick.