In Australia, the police advise to call 000 (emergency line) only if:
  • Someone is obviously hurt, or if there's a chance of a head wound or shock
  • There's over $500 worth of damage to the cars.
As the above write-ups state, exchange names, licence numbers, car license plates, and all that jazz. Get the contact names and numbers of witnesses, if possible, and write down your version of events quickly. They'll come in handy later.

It is wise to get the contact details of the other driver even if the damage seems minor. You might not immediately know if the fender-bender has cracked the radiator or done some other damage, and it could be expensive later. Of course, if the accident's your fault and the other person doesn't ask for your name or number, get out of there! (Obviously, this is the irresponsible course of action.)

An anecdotal hint*

If you are ever in an accident where: you can leave the car and return home by another means of transport, and go and report the accident in the morning/ the next day. It is possible to claim that you had a head injury and were disoriented and merely wanted to go home, and came in to report it as soon as possible. You can't be reliably tested for drugs or alcohol (as long as you weren't drinking/ smoking pot hours before you went in) and are home free.

You shouldn't be charged with 'leaving the scene of an accident' or anything like that unless the officer in charge of the report is very lame. Of course, IANAL and anything is possible, so this is merely anecdotal.

* I was told this by a police officer, in earnest. He didn't seem to be joking, anyway.