For Australian audit partners, methodology leads and anyone preparing for the Group 2 wave.
Australia did not wait. While the US rolled back, the EU simplified and Canada paused, Australia passed its climate reporting legislation in September 2024 and switched on mandatory climate reporting from 1 January 2025. The AUASB approved ASSA 5000 and ASSA 5010 in January 2025, effective almost two years before the international ISSA 5000 takes effect. The Corporations Act now requires the financial statement auditor to provide assurance over the climate report. And reasonable assurance arrives in full for financial years starting on or after 1 July 2030, which is closer than it feels.
This is the most concrete mandatory climate assurance regime in any major market. Here is what it means for how you run an audit.
The structure, in plain terms
Three groups, three start dates, one destination.
Group 1, the largest entities and significant emitters, started reporting for financial years beginning on or after 1 January 2025. The first mandatory AASB S2 reports are already out, and early reviews show what you would expect: wide variation, significant effort and reporting capability being built in real time.
Group 2 starts for financial years beginning on or after 1 July 2026. If that is your client base, you are planning those engagements now.
Group 3 follows for financial years beginning on or after 1 July 2027.
The reporting standard is AASB S2 Climate-related Disclosures. AASB S1, on broader sustainability information, is voluntary, with AASB S2 drawing in the parts of S1 that are relevant to climate. Both mirror the ISSB global baseline.
The assurance standard is ASSA 5000, effective for engagements on sustainability information for periods beginning on or after 1 January 2025, with the phasing set out in ASSA 5010. Limited assurance applies in the early years, moving to reasonable assurance by 2030. The early scope is narrower, covering governance, strategy, and Scope 1 and 2 emissions, and it widens year by year.
A few features matter for planning. The firm that audits the financial statements also provides the assurance. Direct assistance from internal auditors on the sustainability engagement is prohibited, consistent with the existing Australian rule for financial audits. And ASAE 3410 still applies to separate NGER greenhouse gas engagements for now.
Two fronts, now in one file
Climate risk now reaches the Australian auditor in two places that used to feel separate.
Front one: the financial statement audit. Nothing new in principle. Australian Accounting Standards are equivalent to IFRS, so the touchpoints are familiar. Impairment under AASB 136, where recoverable amount must reflect transition and physical risk through forecast cash flows, growth, asset lives and discount rates, which weighs especially heavily on the resources, energy and agricultural sectors. Provisions under AASB 137, where restoration and decommissioning obligations can grow with tighter regulation and where public net zero commitments can crystallise constructive obligations. Decommissioning capitalised under AASB 116 and remeasured over time. Deferred tax recoverability under AASB 112 in businesses in transition. Expected credit losses under AASB 9 reflecting forward looking climate scenarios. And going concern under AASB 101 and ASA 570.
The toolkit is the ASA standards, which mirror the international ISAs. ASA 540 on estimates is central, because climate effects almost always arrive as estimates. ASA 315 drives the risk assessment. ASA 720 on other information matters more than ever, now that a mandatory climate report sits next to the financial report.
Front two: the ASSA 5000 engagement. This is where things change. Much of the data originates outside finance, in operational, engineering and ESG systems with weaker controls. Scope 3 and financed emissions rest on estimates and third party data of uncertain quality. The level of assurance and the components in scope change each year as ASSA 5010 phases in, so a scoping error compounds. And the move from limited to reasonable assurance means the work, and the firm’s exposure, rise every year through 2030.
The real risk is treating this as an extension of the financial audit team’s existing skills. It is a different discipline.
What ASIC is signalling
ASIC has been clear that it will police sustainability disclosure. The temporary modified liability for certain forward looking climate statements, such as Scope 3 emissions and scenario analysis, covers only reports for financial years starting before 2028. After that, the window in which directors and their auditors enjoy softer liability closes, and the standard expected of preparers and assurers rises as 2030 approaches.
Six actions before the next engagement
- Scope precisely, every year. ASSA 5010 moves the goalposts annually: the components in scope and the level of assurance both change. Build this into the engagement letter and acceptance process instead of reusing last year’s scope.
- Walk the data before you plan the work. Where does the Scope 1 figure come from? Who owns the Scope 2 calculation? What third party dataset sits under Scope 3? If the answer is unclear in the first year of limited assurance, it will fail when reasonable assurance arrives.
- Decide on specialist skills now. Carbon accounting, environmental science and scenario analysis are not part of the standard audit skill set. Build in house, use experts under the relevant standard, or both, and document your evaluation.
- Use ASA 720 on the financial statement audit. The AASB S2 report sits right next to the financial statements. If the impairment model is buoyant and the climate narrative is cautious, that is audit evidence, not an awkward coincidence.
- Watch independence. If your firm advises the client on strategy and transition and then provides assurance over the climate report, that is the same self review threat the profession already manages for financial audits. The new sustainability ethics requirements address it. Apply them.
- Document the challenge, not just the conclusion. Inspection files that show a conclusion without the work behind it are the ones that get written up.
The bottom line
Mandatory climate assurance in Australia is not a compliance exercise with a soft landing. The phasing from limited to reasonable assurance through 2030 only goes one way, the modified liability period is time limited and the regulator is paying attention. The firms that will do well are already investing in specialist skills, scoping each year’s engagement carefully and treating the link between the climate report and the financial statements as a primary source of audit evidence.
The ones that do not will discover, somewhere around 2028, that reasonable assurance was always coming and nobody built for it.
Based on the Corporations Act 2001 as amended, AASB S2, ASSA 5000 and ASSA 5010 (approved January 2025) and ASIC guidance on sustainability reporting. This is professional commentary, not legal or audit advice. Verify the current standards and timetable before acting on a specific engagement.