Cumulative impact assessment

12 April 2022

What is cumulative impact assessment?

Cumulative impact assessment (CIA) is designed to fill a gap in business-as-usual, project-based environmental impact assessments (EIAs). It extends the EIA approach to consider multiple interacting, and often compounding, impacts to values (environmental, social and economic). In extending the scope from a single impact to cumulative impact, a regional perspective is often needed, as impacts can be spatially dispersed and have various interactions or spatial dependencies.

In south-eastern Australia, a classic example of cumulative impact is the risk of bird or bat collisions with wind-power turbines. While an individual wind farm might not pose an unacceptable risk, the cumulative effect of multiple developments can add up to an unacceptable impact. For example, improving part of the habitat of the critically endangered orange-bellied parrot (Neophema chrysogaster) was considered insufficient because of the multiple challenges the birds face when migrating long distances (1).

Given that there are limits on what an individual developer can be asked to assess, CIA is designed to fill the gap and broaden the assessment to understand the risks.

Orange-bellied Parrot

Orange-bellied Parrot, Neophema chrysogaster, Australia. Photo: Quentin J Lang.

Statutory obligations for regional land-use planning

The controls available are strategic environmental assessment (SEA) at the federal level, with some states having capacity for CIA. The federal SEA involves Matters of National Environmental Significance and has the potential for efficiencies in this agreement between proponent(s) and the federal government. However, as stated in the regional land-use planning summary, SEA has been underused.

Western Australia has capacity for CIA under Section 16(e) of its Environmental Protection Act 1986. An example of a project assessed under this section was the 2020 cumulative impact assessment of developments on the environmental, social and cultural values of Exmouth Gulf (2).

How and why is regional planning used?

Evaluating and planning for individual environmental impacts under a standard EIA approach fails to account for multiple compounding and interactive impacts on environmental values. Even with high-quality EIA, we often still observe declines in the values being managed, attributed to ‘death by a thousand cuts’. A regional, strategic approach to EIA to consider these multiple interacting threats is required to address this. This is particularly true where EIAs are being used to identify the need for and requirements of environmental offsets. Ten years of SEA under the EPBC Act have shown that offsets need to be protected early to avoid ongoing decline while the slow processes of large and complex plans are implemented.

Decision-support tools

Regional planning and decision-support tools, such as Marxan, can be tailored to questions of CIA at regional scales. Similarly, environmental assessment tools can be extended to consider multiple and cumulative impacts (3). There are also custom-built programs to spatially map and manage cumulative impacts; however, these have primarily been built for and applied in the marine environment (4). These could be readily translated to terrestrial or coastal-zone land–sea interfaces. There are also custom-built offset calculator tools that could be extended to consider cumulative impacts.

What does success look like?

It is critical that EIA approaches be extended to consider the full spatial scale of impacts and likely dependencies, as well as the interactions with other threats and impacts to values. Successful extension of the EIA approach to a SEA which considers cumulative impacts will ultimately ensure developments occur in the right places with minimum impacts. Similarly, where offsets are needed, it will ensure the correct selection of offsets that reflect the true impacts and losses of environmental values that must be accounted for and offset.

These summaries are works in progress and you can contact Dr Vanessa Adams if you have constructive feedback on these summaries.

Other regional planning approach summaries


  1. Stojanovic D, Potts J, Troy S, Menkhorst P, Loyn R and Heinsohn R (2020) ‘Spatial bias in implementation of recovery actions has not improved survival of Orange-bellied Parrots Neophema chrysogaster‘, Emu-Austral Ornithology, 120(3):263–268.
  2. Western Australia Environmental Protection Authority (2021) Potential cumulative impacts of the activities and developments proposed for Exmouth Gulf, WA EPA website.
  3. Nicol S and Chadès I (2017) ‘A preliminary approach to quantifying the overall environmental risks posed by development projects during environmental impact assessment‘, PLOS ONE, 12:e0180982.
  4. Bonnevie IM, Hansen HS and Schrøder L (2020) ‘SEANERGY – a spatial tool to facilitate the increase of synergies and to minimise conflicts between human uses at sea‘, Environmental Modelling and Software, 132:104808.; and Grech A, Hanert E, McKenzie L, Rasheed M, Thomas C, Tol S, Wang M, Waycott M, Wolter J and Coles R (2018) ‘Predicting the cumulative effect of multiple disturbances on seagrass connectivity‘, Global Change Biology, 24:3093-3104.

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