Purely off market size, it is somewhat to be expected that 2D doesn't have the best network everywhere.
Different networks tend to have their "good" area, where they have focused networks.
awhile back, it used to be spark for rural, Vodafone for City. 2D has some decent stuff down south.
Oddly enough, this includes areas where 2degrees roams onto Vodafone. Both have coverage so it's not a locked out site, but rather Vodafone just has a better signal and works a bit better.