Seems to be the default speed limit through any roadworks (of which there are a lot) in NZ whereas in WA it's typically 80kph (highways), 60kph or 40kph if they're actively working.
We only seem to have a very few 40kph areas outside of said roadworks and they're typically heavily pedestrian oriented (Rockingham foreshore and Vic Park along Albany Hwy to name a couple).
Not many NZ drivers seem to pay attention to the 30 signs though.
You'd never get anywhere if you did
Not many drivers anywhere pay attention to roadwork speed limits.
The whole Melbourne CBD, I think, is now a 40 zone, as are a lot of suburban shopping strips in Melbourne. The logic is that in areas with high pedestrian density, the difference in speed between 40 and 60 makes a huge difference to the survival chances of a pedestrian hit by a car (I think it's something like 70% at 40, 10% at 60).