Once the Crown proves the elements of a regulatory or provincial offence or otherwise establishes a duty upon the accused, the onus is on the accused to establish due diligence. This must be proven on a balance of probabilities. The conduct of the accused is measured against that of "a reasonable person in similar circumstances".
This defence is, by definition, only applicable to "strict liability" offences and not "absolute liability" offences.
e.g. R v Ariganello, 2013 ONCJ 13 (CanLII)
R v Pontes, 1995 CanLII 61 (SCC),  3 SCR 44, at para 32
Lévis (City) v Tétreault; Lévis (City) v 2629-4470 Québec inc., 2006 SCC 12 (CanLII),  1 SCR 420 at para 15 ( “the accused in fact has both the opportunity to prove due diligence and the burden of doing so. An objective standard is applied under which the conduct of the accused is assessed against that of a reasonable person in similar circumstances.”)
Pontes, at para 32
see also Regulatory and Provincial Offences
Due diligence requires the accused to "take all reasonable steps" or "all reasonable care" to avoid the harm that resulted. 
Due diligence defence is also available where the accused "had an honest but mistaken belief in facts which, if true, would render the act innocent".
It is not necessary that the accused take all conceivable steps, however.
In assessing due diligence, WD test for credibility does not apply.
This is considered from the perspective of a reasonable person in similar circumstances.
This does not mean that the accused would be required to perform acts that would put the accused in unreasonable danger.
R v British Columbia Hydro and Power Authority, 1997 CanLII 4373 (BC SC),  BCJ No. 1744 (S.C.), at para 55 ("In other words, an accused must take all reasonable steps to avoid harm. However, that does not mean an accused must take all conceivable steps.")
Pontes, at para 32
Pontes at para 32
- BC Hydro and Power Authority at para 55
R v Carpentier 2005 MBCA 134 (CanLII) at para 27
Ariganello at para 18
R v Defaria  O.J. No. 5427, 2008 ONCJ 687 (CanLII) at para 16
- La Souveraine, Compagnie d'assurance générale v Autorité des marchés financiers, 2013 SCC 63 (CanLII), a accused can "avoid liability by showing that he or she took all reasonable steps to avoid the particular event .... The defence of due diligence is based on an objective standard: it requires consideration of what a reasonable person would have done in similar circumstances.")
Industry Standards and Practices
Compliance with industry standard practice is a factor that weighs in favour of due diligence but it not determinative.
Industry standards and practice cannot be of any value to due diligence if it is established that the standard is insufficient. It then cannot be used as a shield against responsibility.
see R v Emil K. Fishing Corp.; R v Kukuljan, 2008 BCCA 490 (CanLII),  BCJ No. 2326; 304 D.L.R. (4th) 725 leave refused
R v Bui,  O.J. No. 1456(C.J.)
R v London Excavators & Trucking Ltd., (1997) 26 C.C.E.L.(2d) 132 (Ont. P.C.)(*no CanLII links)
R v Pilen Construction of Canada Ltd.  O.J. No. 5650, affd  O.J. No. 2980 (S.C.J.)(*no CanLII links)
Libman, Regulatory Offences in Canada, at page 7-27 (“…a defendant cannot hide behind commonly accepted standards of care if, in the circumstances, due diligence warrants a higher level of care.”)