Best coverage will almost always be Telstra, best rate by definition
will be comeone else.

If you are in the 'outback', and want service, outside of the Alice
Springs, Darwin, Kathrine(?) and Ayers Rock, you need to Telstra
prepaid CDMA phone. There is no GSM service is the small remote
communities.

I've had really bad experiences with GlobalStar sat phones in the
outback. It may be an anywhere phone, but it isn't an anytime phone!

These questions are not easy to answer.

Generally, Telstra has the best coverage.  But you will find
places where Optus or Vodafone (the other 2 carriers who
operate GSM networks) have coverage while Telstra does not.

With one very minor exception (Vodafone onto Telstra in parts of
Victoria), there is no roaming arrangement between Australian
GSM networks.

Vodafone has full coverage on a few major Australian highways
because they won a government contract to provide it.  Telstra
and Optus have both declined the government-guaranteed offer of
roaming onto those cells.  Generally, Telstra and Optus have
fair coverage on major highways anyway.  Other than those
highways, Vodafone rural coverage is the poorest of the 3
carriers.

Be aware that standard GSM is technically limited to 35 km range
from the nearest cell.  There are very few extended-range
cells.  While more populated areas have coverage, the vast
majority of the continent has none.

Telstra CDMA has significantly greater coverage than any GSM
network, if you want to consider the possibility of buying a
pre-paid CDMA phone when you get here.  Optus also sells CDMA
pre-paid phones which operate on Telstra's CDMA network.

Pricing gets even more difficult to cover adequately.  This is
not helped by different call prices applying to different $
amounts of top-up with some SIMs.  Also, some pre-paids let you
choose between different rates (cheap SMS with expensive voice
calls, or vice versa).  Comparing like-for-like is difficult
and time-consuming.

Virgin Mobile is a significant reseller of Optus network
capacity, using their own SIMs.  The URLs to begin looking at
prices are:

http://www.telstra.com.au/telstraprepaidplus/index.htm

http://tinyurl.com/aa6xj        (Optus)

http://www.virginmobile.com.au/rates/prepaid.html

http://tinyurl.com/mwhwy        (Vodafone)

John