This question comes from a NSW Paramedic who wants to know where their authority comes from when they cross the border into the ACT in order to deliver a patient to an ACT mental health facility.
The answer is that both NSW and ACT law applies. First NSWAS is an ambulance service created by the Health Services Act 1997 (NSW) not the Emergencies Act 2004 (ACT) so the paramedics employment and protocols all come from NSW even when travelling interstate so they have to treat the patient as directed by their employer.
Second, I assume that the patient has been scheduled under the Mental Health Act 2007 (NSW) not the Mental Health Act 2015 (ACT). The NSW Act specifically has interstate application. Section 174(1) says:
A person who may be taken to and detained in a mental health facility under Chapter 3 or under the Mental Health (Forensic Provisions) Act 1990 may be taken to a mental health facility in another State instead of a mental health facility in this State, if this is permitted by or under a corresponding law of the other State.
The relevant ACT law says at s 255:
A person apprehended in another State under a corresponding law may be taken to an approved mental health facility in the ACT by an authorised officer or interstate authorised person if the authorised officer or person believes on reasonable grounds that being taken to an approved mental health facility in the ACT is—
(a) in the best interests of the safe and effective treatment, care or support of the person ; and
(b) allowed under a corresponding law.
So that section says that an authorised officer (which I assume, without double checking includes a NSWAS paramedic) can transfer the patient into the ACT if that’s allowed under NSW law, which it is.
The authority of NSW paramedics to transfer a patient to the ACT is confirmed by the Mental Health (Treatment and Care) (Interstate Application of Mental Health Laws) Agreement 2003 between the ACT and NSW.
Conclusion
The answer to the question is that both the NSW and ACT laws apply because the laws say they do. The NSW law says the patient can be transferred to the ACT and the ACT law says the same and points to the authority in the ‘corresponding law’ (ie the NSW law). So if a person (a paramedic) has the authority under NSW law to transport and treat a person with a mental health emergency they can transport them to, and treat them in, the ACT until they hand them over to the ACT mental health services.
Australian Emergency Law 
Thanks Michael,
I assume your discussion above concerns a patient who was initially assessed, and the decision made while in NSW, to transport them to a mental health facility interstate. What if the paramedic crew was in the ACT (returning to NSW post a previous case) and was flagged down in the ACT. Can the NSW paramedic use their NSW authority to detain and transport this patient (assuming it is indicated)?
Indeed I was asked to address the question in the context of ‘a patient who was initially assessed, and the decision made while in NSW, to transport them to a mental health facility interstate’. If the scenario you’ve described the Mental Health Act 2007 (NSW) will have no application, the relevant law will be in the Mental Health Act 2015 (ACT). Section 80 says:
Assuming that the NSW Paramedic has not been authorised under the ACT Act, a NSW paramedic finds a patient in those circumstances may rely on common law principles (necessity) and statutory authority (Crimes Act 1900 (ACT) s 18 “It is lawful for a person to use the force that is reasonable to prevent the suicide of another person or any act that the person believes on reasonable grounds would, if committed, result in the suicide of another person”) to detain the person pending backup from ACT Policing or ACT Ambulance.