Reasonably Practicable in WHS Law: The Complete Guide

What 'Reasonably Practicable' Means in Section 18 of the WHS Act

Section 18 of the Work Health and Safety Act 2011 defines what is 'reasonably practicable' in relation to a duty under the Act. The definition requires weighing the likelihood of the hazard or risk occurring, the degree of harm that could result, what the duty holder knows or ought reasonably to know about the hazard and ways of eliminating or minimising it, the availability and suitability of ways to eliminate or minimise the risk, and after assessing all of the above, the cost of eliminating or minimising the risk.

The critical point is that this is not a question of what was done. It is a question of what was required. A duty holder who argues that they followed their usual practice, or that no other employer in the industry did anything different, does not automatically satisfy the reasonably practicable standard. The standard looks forward: what should have been done, given what the duty holder knew or ought to have known.

The same definition appears in the WHS Acts of the Commonwealth, NSW, Queensland, South Australia, Western Australia (from 31 March 2022), the ACT, the Northern Territory and Tasmania. Victoria's Occupational Health and Safety Act 2004 defines the term in section 20(2) using the same five matters, and WorkSafe Victoria applies a clear presumption in favour of safety when weighing them.

The Five Factors Weighed Under Section 18

1. Likelihood of the Hazard or Risk Occurring

The first factor is the probability that the hazard or the risk will materialise. A rare and unpredictable risk requires less effort to control than a common and foreseeable one. But likelihood alone does not determine the duty. A low-probability event that could cause catastrophic harm still attracts serious obligations.

2. Degree of Harm

The severity of injury that could result if the hazard is not controlled. Falls from height, for example, carry a high degree of potential harm even where the probability of a fall on any given day is low. The combination of likelihood and severity creates the risk level that must be controlled.

3. Knowledge of the Hazard and Controls

What the duty holder knew, or ought reasonably to have known, about the hazard and the ways of eliminating or minimising it. Published codes of practice, industry standards and Safe Work Australia guidance are all relevant to what a duty holder is taken to know. A duty holder cannot avoid liability by claiming ignorance of a hazard that is well documented in their industry.

4. Availability and Suitability of Controls

Whether ways to eliminate or minimise the risk were available at the time and suitable for the specific work environment. Controls that existed but were not used are relevant to breach. Controls that were not yet available or not yet proved effective for the specific application are relevant to what was reasonably practicable at the relevant time.

5. Cost of Eliminating or Minimising the Risk

Cost is the last factor considered. Section 18(e) only brings cost in after the extent of the risk and the available ways of eliminating or minimising it have been assessed, and asks whether the cost is grossly disproportionate to the risk. A high-cost control is still required if the risk is serious enough. Safe Work Australia's guidance makes the same point: cost can only rule out a control where it is grossly disproportionate to the risk, which is rare where there is a risk of serious injury.

Foreseeability and Reasonably Practicable

Foreseeability is built into section 18 through the knowledge factor: what the duty holder knew, or ought reasonably to have known, about the hazard and the ways of controlling it. A duty holder cannot be expected to have controlled a risk it could not reasonably have known about. In civil negligence claims, foreseeability is also a separate threshold question under the civil liability legislation, for example section 5B of the NSW Civil Liability Act 2002.

For more detail on foreseeability as a standalone element, see foreseeability in WHS litigation.

How an Expert Witness Analyses Reasonably Practicable

A WHS expert witness applies the section 18 framework to the specific facts of the matter. The analysis identifies which hazard or risk is in issue, assesses the likelihood and potential severity of harm from that risk, identifies the controls that were available and suitable at the time, and assesses whether the duty holder implemented those controls. Where controls were not implemented, the analysis addresses whether the cost or impracticability of those controls provided any legitimate reason for not implementing them.

Expert opinion on reasonably practicable is prepared as part of a formal expert witness report or a breach analysis and liability assessment. In WHS prosecutions, it usually goes to whether the control measures the prosecution relies on were available and suitable at the time.

Common Misconceptions About Reasonably Practicable

PCBUs and some legal practitioners hold misconceptions about what the reasonably practicable standard requires. The most common are that industry practice defines the standard, that cost alone defeats the duty, that a written procedure satisfies the obligation, or that an absence of prior incidents means no duty existed. None of these positions is correct. The standard requires active identification of hazards, genuine consideration of available controls, and implementation of those controls, not merely documentation that they were considered.

For related context see duty holder obligations under the WHS Act and the hierarchy of controls explained.