(1) except as provided (2), a person who has been directed by a peace officer to assist in making an arrest or to prevent an escape from custody is justified in using physical force when and to the extent that they reasonably believe that force is necessary to carry out the peace officer's direction.

(2) a person who has been directed to assist a peace officer under circumstances specified in (1) may use deadly physical force to make an arrest or to prevent an escape only when:

(a) they reasonably believe deadly force is necessary for self-defense or to defend a third person from what they reasonably believe to be the use or imminent use of deadly physical force OR

(b) they are directed or authorised by the peace officer to use deadly physical force unless they know that the peace officer is not authorised to use deadly physical force under the circumstances