The measure is aimed at reducing accidents in urban areas with heavy pedestrian traffic and narrow streets, marking a major shift in the country’s traffic rules as these roads account for around 70% of all ordinary roads nationwide.
[...] above [30 kph] the risk of pedestrians suffering fatal injuries in accidents is said to spike.
[...]
The aim of the speed limit change “is to prevent accidents by stopping dangerous driving, such as failing to slow down even on busy streets”
40km/h has been the minimum speed limit on public roads in Aus for quite a while, and it's still the norm. 30 is pretty new and will probs take some time to adjust to.