Sustainability Report, Turnover, Accounting Firm Size, Corporate Tax Nexus and Financial Performance of Listed Indonesian Coal Companies

The financial performance of coal businesses listed on the Indonesia Stock Exchange (IDX) between 2021 and 2023 is examined in relation to sustainability reporting and public accounting firm size. The corporate tax turnover ratio (CTTOR) is used as an intervening variable, while gross profit margin is used as the dependent. Using SPSS software, multiple linear regression analysis is performed on data from 96 out of 32 organisations. With scores of significant levels 0.015 and 0.000, the results of the first hypothesis test accept and show that the size of public accounting firms and sustainability reporting have a significant impact on gross profit margin. This means businesses that are audited by Big Four firms and those that publish sustainability reports typically have higher profitability. The second hypothesis reveals that CTTOR is not impacted by sustainability reporting, which received a score of 0.933, but it is by public accounting firm size and gross profit margin; both have a significant level of 0.000. The second hypothesis is thus disproved, indicating that compared to sustainability disclosure alone, audit quality and operational success have a stronger correlation with tax effectiveness. There is some evidence to support the third theory. The results of the mediation study show that sustainability reporting has a significant mediating impact since its indirect influence on CTTOR through gross profit margin (0.1128) is larger than its direct effect (0.007). On the other hand, there appears to be limited mediation, as the direct effect of audit firm size on CTTOR (0.368) is greater than the indirect effect. In conclusion, audit firm size directly contributes to greater financial transparency, whereas sustainability reporting indirectly improves tax efficiency through increased profitability. To more accurately evaluate sustainability quality, future research should broaden the sector focus, prolong the study period, and use standardised frameworks like the Global Reporting Initiative (GRI).