Due diligence is a legal concept. From the Concise Australian Legal Dictionary, due diligence is, a minimum standard of behaviour involving a system which provides against contravention of relevant regulatory provisions and adequate supervision ensuring that the system is properly carried out. The concept of due diligence has been captured in Corporations law, Environmental Law and now the model Work Health and Safety legislation.
In an engineering and technological context due diligence is often construed to just mean compliance. This is not the case.
Due diligence is an aspect of moral philosophy, that is how the world ought to be and how humanity ought to behave to achieve it. Often, this is along the lines that one should treat others as you would like to be treated by them.
Due diligence for safety in design purposes uses the principles behind the judgments of the courts and applies them pre-event to ensure sound decision making. That is, reverse engineering of judicial decisions as shown above.
How this manifests itself is that sometimes bad things happen and Courts and Royal Commissions question the design and design process with the advantage of hindsight to see what could have been in place, if it had been in place would have stopped the bad thing / accident from happening. Outcomes from such investigations are then fed back to see if the parties involved in the design were diligent.
Safety due diligence is all about showing pre-event that the operation is safe. This means that when safety is being considered in the design process, all of the people that could be affected by the design need to be taken into account and the potential hazards to which they are exposed, over the entire lifecycle.
From a safety due diligence aspect, all reasonable practicable precautions need to be in place applying the hierarchy of controls.