The following should be a simple question from an RFS volunteer, but it requires quite a long answer. The question:
… relates to a witness to a motor vehicle accident .
The scenario is: You are driving along a road (say in NSW, my home State) and witness a fairly serious motor vehicle accident. You stop and render what assistance you can i.e. give first aid, calling police and ambulance, trying to protect the incident from other traffic. The police arrive and start to do their usual thing of managing traffic, examining the accident scene, breath testing and interviewing the driver(s) of the vehicle(s) involved in the accident, and interviewing witnesses. You approach the police person and advise him/her that you were a witness but have an important appointment elsewhere so cannot stay any longer. You offer the police person a business card or piece of paper with your name and contact details.
My question: Can you, not having been involved in the accident only a witness, leave the scene when you wish or can the police direct you to remain at the scene until they get around to interviewing you?
The short answer is that you can leave the scene when you wish and no, the police cannot direct you to remain at the scene until they get around to interviewing you. The long answer is more complex.
An arrest occurs when police make it clear that a person is no longer free to go about their business (R v O’Donoghue (1988) 34 A Crim R 397, 401). Although it is usual to indicate an arrest by touching the person and saying ‘you’re under arrest’, there is in law ‘… no magic formula; only the obligation to make it plain to the suspect by what is said and done that he is no longer a free man’ (R v Inwood  2 All ER 645, 649; see also Alderson v Booth  2 QB 216, 220).
If the police ‘direct you to remain at the scene’ then they are communicating to you that you are no longer free to go about your business and that is an arrest.
Purpose of arrest
‘At common law, the only legitimate purpose of an arrest was to bring a person before a court to answer an allegation of wrongdoing. It was impermissible to arrest a person simply to allow police to ask questions (Bales v Parmeter (1935) 35 SR (NSW) 182) or for the purpose of investigating that person’s possible involvement in a criminal offence.’ (see Michael Eburn, Rod Howie and Paul Sattler, Hayes and Eburn Criminal Law and Procedure in NSW (2013, 4th ed, Lexis/Nexis), [11.46]; Williams v R (1986) 161 CLR 278 and Law Enforcement (Powers and Responsibilities) Act 2002 (NSW) s 99).
In Collins v Wilcock  3 All ER 374 two police officers observed a women who they suspected of ‘soliciting for the purpose of prostitution’. The officers asked the woman to get into the car so they could ask her questions, which she refused to do and walked away. A police officer got out of the car and followed her order to question her. Again the woman refused to speak to the police and continued to walk away. The police officer then took hold of her arm in order to stop her. The woman swore at the officer and scratched the officer’s arm. She was convicted of assaulting a police officer in the execution of her duty. On appeal the conviction was set aside. Robert Goff LJ said:
A police officer has no power to require a man to answer him, though he has the advantage of authority, enhanced as it is by the uniform which the state provides and requires him to wear, in seeking a response to his inquiry. What is not permitted, however, is the unlawful use of force or the unlawful threat (actual or implicit) to use force and, excepting the lawful exercise of his power of arrest, the lawfulness of a police officer’s conduct is judged by the same criteria as are applied to the conduct of any ordinary citizen of this country.
Arrest to establish identity
There are some circumstances where a police officer can detain a person in order to establish their identity. The Law Enforcement (Powers and Responsibilities) Act 2002 (NSW) s 11 says:
A police officer may request a person whose identity is unknown to the officer to disclose his or her identity if the officer suspects on reasonable grounds that the person may be able to assist in the investigation of an alleged indictable offence because the person was at or near the place where the alleged indictable offence occurred, whether before, when, or soon after it occurred.
The police may also require proof of identity (s 19). Failure to comply is an offence (s 12) and so could justify an arrest (s 99) and the use of force to ensure compliance (ss 230 and 231).
Applying those principles to the given scenario, requiring a person to remain at the scene until police get around to interviewing them would be an arrest. Because the purpose of the arrest is to ask questions, not to bring the person before the court to face an allegation of wrongdoing, it would be an unlawful arrest. Section 11 of the Law Enforcement (Powers and Responsibilities) Act 2002 (NSW) does not allow detention to ask questions and would not be relevant in the given scenario as my correspondent was volunteering proof of their identity so the police could locate them later in order to complete their investigation.
This brings us full circle back to the short answer: you can leave the scene when you wish and no, the police cannot direct you to remain at the scene until they get around to interviewing you.