Land borders can be closed except for a few checkpoints, which serve as the equivalent of international airports (and seaports, don't forget about those). I don't know much about Australian geography, so I had to check on a map, but it looks like it's possible to travel from Melbourne to other parts of Australia over land. Apparently some measures were taken to prevent the virus from spreading along that route.

Driving case counts down to zero isn't useless since any outbreaks that flare up again due to gaps in border security will be localized and can be kept in check with localized lockdowns instead of shutting down the whole country.