If this defence is being raised in relation to a murder charge, the emergency must involve a risk of death or really serious injury (section 9AI(3)).
Section 9AI of The Crimes Act 1958
(1) A person is not guilty of a relevant offence in respect of conduct carried out by him or her in response to circumstances of sudden or extraordinary emergency.
(2) This section applies if and only if the person carrying out the conduct reasonably believes that—
(a) circumstances of sudden or extraordinary emergency exist; and
(b) committing the offence is the only reasonable way to deal with the emergency; and
(c) the conduct is a reasonable response to the emergency.