What Limited and Reasonable Assurance Mean for Sustainability Reporting

Mandatory climate reporting under AASB S2 requires external assurance. This is one of the most significant differences between mandatory sustainability disclosure and the voluntary frameworks many Australian businesses have used previously — CDP, GRI, and similar voluntary reports are not subject to statutory assurance.

Understanding what assurance actually involves — and how requirements differ between limited and reasonable assurance — is important for preparing data and setting expectations internally.

The assurance standards

The Auditing and Assurance Standards Board (AUASB) published two standards for sustainability assurance:

ASSA 5000 — Complete Subject Matter Sustainability Assurance Engagement is the primary standard. It covers assurance over sustainability information generally, including both limited and reasonable assurance engagements.

ASSA 5010 — Assurance of Scope 3 Greenhouse Gas Emissions addresses the specific challenges of providing assurance over Scope 3 value chain emissions, which have distinctive data challenges.

These standards align with international developments in sustainability assurance and are closely related to the IAASB’s ISSA 5000.

Limited assurance: the starting point

Limited assurance is what most entities will start with. The conclusion is expressed in negative form:

“Based on the procedures performed, nothing came to our attention that causes us to believe the sustainability report is not prepared, in all material respects, in accordance with the applicable criteria.”

This is sometimes misread as a weak conclusion. It isn’t. Achieving a clean limited assurance conclusion requires:

• A documented and appropriate methodology
• Actual emissions data that can be traced to source documents for a tested sample
• Reasonable completeness — all material emission sources accounted for, with gaps disclosed
• Appropriate emission factor selection with version references
• A functioning internal review process

Where limited assurance is lighter than reasonable assurance is in the extent of procedures. The assurance provider tests a sample of transactions rather than seeking broader coverage, uses more inquiry-based procedures, and does not need to obtain sufficient evidence for a positive conclusion.

Reasonable assurance: the higher standard

Reasonable assurance is expressed in positive form:

“In our opinion, the sustainability report is prepared, in all material respects, in accordance with the applicable criteria.”

Achieving a clean reasonable assurance conclusion requires substantially more evidence:

• More extensive transaction testing (larger samples, broader coverage)
• More rigorous validation of methodology and boundary completeness
• Substantive analytical procedures to identify anomalies
• Stronger evidence on completeness — not just that sampled items are correct, but that the population is complete

Reasonable assurance also requires more mature internal controls — processes that systematically prevent and detect errors, not just a review at period end.

The phase-in timeline

Australia’s mandatory regime phases in assurance requirements over time. The specific transition dates are part of the ASRS framework:

• Year 1: Limited assurance for all groups
• Year 2+: Transition timeline toward reasonable assurance (details specified in the Corporations Act regulations)

The intent is to allow the market — both reporting entities and assurance providers — to build capability before the more demanding standard applies. The assurance market in Australia is developing: the major audit firms have sustainability assurance practices, and specialist sustainability assurance providers are growing.

What gets assured?

In the early years of mandatory reporting, assurance is focused on:

• Scope 1 and Scope 2 greenhouse gas emissions (the mandatory quantitative disclosures)
• Compliance with the AASB S2 disclosure requirements (including governance, strategy, and risk management)

Scope 3 emissions, when they become mandatory, will be subject to assurance under the ASSA 5010 standard — which acknowledges the greater inherent uncertainty and methodological challenges in Scope 3 data.

Implications for data preparation

The practical question is: what does your data need to look like to achieve a clean limited assurance conclusion?

The minimum requirements are:

1. Source document linkage — every material emissions figure can be traced to the invoice, meter read, or fuel record that supports it
2. Factor documentation — the NGA factor version and reference is recorded for each calculation
3. Methodology document — a written description of the approach, boundaries, and estimation methods
4. Data register — a complete list of all expected data sources, with actual/estimated/missing status
5. Internal review record — evidence that a responsible person reviewed the figures before the report was finalised

If your current process cannot produce these five things, it needs to be upgraded before your first mandatory reporting period.

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Ayika is designed to maintain the documentation trail that makes limited and reasonable assurance achievable. See how the platform supports assurance readiness.