Since you are in NSW, it is worth checking out the DEH minimum cage sizes. These are probably going to be mandated, so best to be safe. The Draft Code of Practice (expected to become law) specifies Adult enclosure for Darwins as:
Length 90 – 150 cm
Width 60 – 90 cm
Floor area 5,400 –13,500 cm2
You are also supposed to provide adequate vertical space for arboreal species, so I'd go with 1 metre tall as a minimum.
You can find the draft code of practice here.