I think all cars should be limited to 5kph and have someone walk in front of them waving a red flag as they did in the early days.
Seriously, I support the 50k limits in suburban residential streets and heavy pedestrian load locations like the CBD, but it needs to be more clearly indicated.
I was back in Adelaide a week before I noticed that the speed limit now drops to 50K as you drive through the parklands into the city.
Sure, there is a sign on each road, but if you are watching traffic in an area where you are no longer totally familiar with the flow one small sign is easily missed. [No I wasn't picked up.]
I like the NSW tendency to paint the speed limit on the road in big white letters every so often.
In Victoria, major suburban roads have white lines along them and are 60K and unmarked roads are 50, while multi-lane major roads are usually 80K. This means there is a system everyone knows, and if you are not sure you can work it out just by looking at the road itself. The draw back there is the 3 kph margin - you have to constantly check your speedo or drive 5K under the speed limit and annoy those behind you.
In broad terms, yes, you can avoid the fines by obeying the law, but I have never intentionally exceeded the speed limit in a city and suburban environment in my life, but have been fined for doing it accidentally by small margins several times. [I admit I can't claim not to have sped intentionally on country highways outside built up areas, where I would still dispute the rationality of current speed limits.]
However, campaigns like the one that claims you are going so much faster at impact if you are only 5K faster when you brake don't help credibility - I've run it through the formulae several times and it just does not compute...