There are two main types of speed limit signs: regulatory speed signs which you must not exceed, and advisory speed signs which indicate a recommended maximum speed for a corner in good driving conditions for the average car.
There are two default speed limits if there are no signs. The default urban speed limit is 50kph, and the default open road speed limit is 100kph.
Regulatory speed signs are always shown as black numbers in a red circle usually in multiples of 10kph, i.e. 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100kph. In other states you may find 110kph limits. However, in some places, particularly on private property, you might see 5kph and 15kph limits, such as this sign in Manly.
Advisory signs have a yellow background and are indicated by speeds ending in 5kph, e.g. 15, 25, 35, 45, 55, 65, 75, 85 and 95. The are usually found before curves, traffic calming measures and crests.
They are usually accompanied by a sign that shows what the hazard is, such as risk of tipping in a high-sided vehicle (as shown in this sign on The Grand Parade in Ramsgate Beach just before the sharp right hander onto Sandringham Street), a curve to the right or left, or a speed bump or crest.
When a speed limit ends and the default speed limit applies, you may see a sign which says END and the speed in a black circle (“end speed limit” sign), or simply a black circle with a diagonal line through it (“speed de-restricted sign”).
Other areas may be subjected to speed limits at specific times such as near schools, or in a shared use zone where there is likely to be a lot of pedestrians as well as vehicles. Shared zones have a speed limit of 10kph. School zones have a speed limit of 40kph.
Road works will have a speed limit to help keep the road workers safe. Local traffic zones will have a 40kph limit for residential side streets to give better safety for pedestrians, and a quieter life for residents.
School buses carrying children have speed limits for overtaking them of 40kph. These are noted on the back of the buses, as per the photo shown.
If the bus headlights are flashing, too, this means they are dropping children off so pay special attention to kids that may dash across the road in front of you.
Variable speed limit signs on tunnels, motorways and bridges show a temporary enforceable limit based on the traffic conditions at the time. The photo shows a temporary speed limit of 90kph on the motorway.