R. v. Mazerolle, 2013 NBPC 21:
3. This defence must be restricted to the rare cases where the action was "involuntary". It must be "strictly controlled and scrupulously limited", as Dickson J. stated at page 250. Therefore, the defence of necessity has limited application. The Court review involves "a pragmatic assessment of the position of the accused, tempered by the need to avoid negating criminal liability on the basis of a purely subjective and unverifiable excuse": see R. v. Ruzic, 2001 SCC 24 (CanLII),  1 S.C.R. 687, at paragraph 61.
4. The elements that must be present for the defence of necessity were outlined in R. v. Latimer, 2001 SCC 1 (CanLII),  1 S.C.R. 3, at paragraphs 28-34. I quote some excerpts:
28 Perka outlined three elements that must be present for the defence of necessity. First, there is the requirement of imminent peril or danger. Second, the accused must have had no reasonable legal alternative to the course of action he or she undertook. Third, there must be proportionality between the harm inflicted and the harm avoided.