Glossary›Assurance, audit evidence and working papers›Reasonable assurance
Glossary term
Cluster A · A5
Tier 1 · differentiator
Reasonable assurance
Definition
Reasonable assurance is the higher assurance level and carries the same level of confidence as a financial statement audit. The practitioner tests controls and substantive detail, then expresses a positive opinion that the information is presented fairly in all material respects. Australian sustainability reports move to reasonable assurance in each entity’s fourth reporting year.
On this page
In practice
The jump from limited to reasonable is not incremental. It changes what has to exist inside the business, not just what has to be handed over. Reasonable assurance requires the practitioner to understand and test internal controls over the emissions data, which means those controls must be designed, documented and operating for the whole period. A control implemented in month eleven cannot be tested for months one to ten.
That is why the fourth-year date matters years in advance. An entity that treats its first three years as a document-gathering exercise arrives at reasonable assurance with no control environment, and the practitioner has to do a very large amount of substantive testing to compensate. It is the biggest single cost step in the regime.
What the assurer does with it
The practitioner selects a control, such as the monthly review and sign-off of site fuel data, and tests whether it operated throughout the period. They also perform substantive testing on the numbers. Two things they reject that limited assurance tolerated are undocumented review (“the finance manager checks it”) and a control with no evidence of having operated: a review with no signature, no date and no exception record. The implication is that evidence of the control matters as much as the control.
Commonly confused with
Absolute assurance. Reasonable assurance is high but not absolute; sampling, judgement and the inherent limits of estimation mean some risk always remains.
Timing and relief
Reasonable assurance over all disclosures applies from the fourth reporting year for each group under ASSA 5010: financial years commencing on or after 1 January 2028 for Group 1, 1 July 2029 for Group 2, and 1 July 2030 for Group 3. Earlier or higher assurance is permitted at any time. The Corporations Act backstop is financial years commencing on or after 1 July 2030.
Sources
1
ASSA 5010 Timeline for Audits and Reviews of Information in Sustainability Reports under the Corporations Act 2001
AUASB
2
ASSA 5000 General Requirements for Sustainability Assurance Engagements
AUASB
3
Review status
Review required
Last reviewed
15 September 2026
Editorial pass, unsigned
Reviewer required
Registered company auditor
Next scheduled review
1 July 2030
Part of
Cluster A, Assurance, audit evidence and working papers
47 terms on what an assurance provider tests, what they accept as evidence, and what a preparer has to be able to produce.
Related terms
The level your first three reporting years sit at
What has to exist and operate before controls can be tested
How to count which reporting year you are actually in
Related questions
What is the difference between limited and reasonable assurance?
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Limited assurance is expressed negatively, that nothing has come to the practitioner’s attention suggesting the information is materially misstated. Reasonable assurance is expressed positively, that the information is fairly presented, and it needs far more testing and costs more. Australia starts with limited assurance and moves to reasonable over time.
Does limited assurance mean they check our numbers?
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Partly. A limited assurance engagement rests mainly on inquiry and analytical procedures, with some testing of source data, rather than a full recalculation of your inventory. The practitioner looks for anything that stands out rather than verifying every number, which is why the conclusion is worded negatively.
Other terms in this cluster
Reasonable assurance