3 claims to be able to roam to telstra when signal is not available but actually this might not happen.
In australia the telcos use coverage of population instead of area to calculate their coverage percentage. The different of coverage between Telstra and Optus maybe only less than 1% of population, but this "less than 1% of population" actually covers huge areas, includes most areas of TAS and NT. In the eastern coast Optus is pretty OK.