Simple geographic isolation does not count as reproductive isolation. Especially in this particular case where the land bridge connected NG and Aus a mere 5,000 years ago.
Populations that are fragmented by environmental changes are of course set up to possibly become new species through accumulative genetic drift or changes in environmental pressures. Genetic drift is not as big of a factor as used to be previously believed, changes in environomal pressure is the major driving force.
If simple habitat fragmentation was enough, then Hoser could describe fifty new Pseudonaja .... oh wait a minute, he already did that. ;-)