Purpose: This thesis aims to understand the nature and role of sustainability assurance (SA) in the context of corporate accountability in the New Zealand (NZ) market. This understanding is developed based on an extensive investigation of the perspectives of sustainability managers (SMs), sustainability assurance providers (SAPs), and stakeholders. The study gives particular attention to understanding the perspectives of multiple stakeholder groups, which is missing in the literature. Stakeholders are overwhelmed with various corporate sustainability (CS) disclosures that are susceptible to greenwashing and there is an urgent need to ensure their integrity and reliability. The Global Reporting Initiative (GRI) recommends the use of SA to ensure the credibility of sustainability reports (SRs), but the practice is growing slowly in NZ and the assurance engagement parties are still determining the nature of best practice. The study contributes to the relevant literature in SA practice, highlighting its weaknesses and suggesting possible solutions to reach an ideal model.

Approach: The study employed an interpretive qualitative approach, based on the philosophical assumptions of constructivism and interpretivism. A thorough understanding of SA practice was acquired through the application of the hermeneutic circle. Data was collected from 35 in-depth semi-structured interviews with three categories of participants: SMs from the NZX-listed companies and Sustainable Business Council (SBC) members, SAPs from accounting and non-accounting firms, and representatives from five different stakeholder groups: sustainability consultants, the External Reporting Board (XRB), the New Zealand Stock Exchange (NZX), non-governmental organisations (NGOs), and financial capital providers.

Findings: The practice of SA exists in the NZ market in the form of a weak model: an internal management practice. It is demanded by management and targeted at management, and focuses on the assessment of the quality of both internal control systems and SR presentations, without assessing sustainability performance per se. This weak model of SA achieves accountability to management, but fails to fulfil the promise of stakeholder accountability. Three fundamental factors are currently causing the practice of SA in NZ to diminish in importance. There is a high demand from internal stakeholders, coupled with a strong desire from accounting firms to gain SA market dominance against limited demand from external stakeholders. The study provides evidence that managerial and professional capture occur at different stages of the assurance engagement.

Originality: To my knowledge, this is the first NZ study to make an extensive investigation into the perspectives of stakeholders. This is also the first NZ study to consider SMs, SAPs, and stakeholders simultaneously in the practice of SA. Considering the three parties of the SA equation together enriched the findings. These findings elicited a list of country-specific factors that are unique to the NZ market, which help in the interpretation of the weak model of SA.

Practical implications: The findings of this study will prove beneficial to SMs, SAPs, and stakeholders. The proposed ideal model of SA practice offers detailed guidance for the three parties on specific mechanisms to achieve stakeholder accountability, instead of management accountability. It may assist them in developing new approaches before, during, and after the assurance engagement in order to avoid falling back into the weak model of SA. The depiction of the two contrasting models of SA practice together provides powerful insights for practitioners and researchers.

Theoretical implications: The study helps to clearly visualise the weak model of SA practice, identify its causes, and suggest future practical steps that should be taken to turn the practice into its ideal shape. Furthermore, new insight is added through the application of Michael Power’s work in the interpretation of the findings.