The speed limit from the time you pass an Accident sign until you have passed the crash site is 20kph.
There is only a compulsory 20kph speed limit if you pass an accident sign. A road user may have put a warning triangle out to advise other road users that there could be an accident ahead, but this is only advisory - you should still slow down, though.
Police will put an accident sign up and it means that there could be emergency services workers on the road (fire, police or ambulance), sharp debris that could puncture your tyre, injured people on the road, power lines or other dangers blocking your path, mud on the road, vehicle fluids on the road, and so on. 20kph is a safe speed for you to be able to stop very quickly if you need to.
You can also check out the 'What to do if you come across an accident' section of our comprehensive article on dealing with crashes and accidents here.