A really good question. From a very quick and dirty search it seems, in relation to an observer rather than a person involved in the accident:
"Under common law, there is no legal duty on any individual, regardless of whether he or she is a doctor, to rescue where there is no prior relationship. However, there are some exceptions to the general presumption that there is no legal obligation to provide emergency aid as a Good Samaritan."
In the Northern Territory, Section 155 of the Criminal Code states:
Any person who, being able to provide rescue, resuscitation, medical treatment, first aid or succour of any kind to a person urgently in need of it and whose life may be endangered if it is not provided, callously fails to do so is guilty of a crime and is liable to imprisonment for seven years.
I think most people will do what they think is the right thing at the time.
Perhaps we have some retired legal gurus amongst us who can shed some light on this.