FWIW, we've just done Cairns to Sydney
using Vodafone prepaid. We knew that Telstra had better coverage, but couldn't stand their rapacious prepaid rates. To our surprise, we had a connection in all but a very few anchorages
, and often up to 20 miles offshore
. Not perfect, but better than we expected.
PS We used the Vodaphone dongle on a 4 M USB cable run out through a dorade and hanging from the boom when in low signal areas. Some sort of preamp would likely improve things.