Climate transition disclosure has expanded rapidly in recent years, yet little is known about how such reporting practices evolve during the early stages of institutionalisation and whether firms converge toward common disclosure patterns. This study examines the development of corporate climate transition disclosure in Australia over the 2020-2023 period, prior to the introduction of mandatory climate reporting. Using a self-developed climate transition disclosure index covering climate ambition, transition preparedness, and transition control, the analysis traces both temporal trends and cross-sectional convergence in reporting practices. The results show that climate transition disclosure increases substantially over time across all three dimensions, with the most pronounced growth occurring in high-visibility commitment and governance-related disclosures. Cross-sectional dispersion declines significantly, indicating increasing convergence in reporting practices. However, convergence is selective rather than uniform. Standardised and externally codified disclosures, such as framework adoption, governance structures, and core emissions metrics, converge rapidly. In contrast, disclosures requiring advanced internal capabilities, including scenario analysis, internal carbon pricing, and capital allocation planning, remain heterogeneous. Heterogeneity analyses further show that larger firms, high-emitting, and firms in climate-sensitive industries disclose more extensively, although gaps narrow over time in several dimensions. Differences associated with financial performance are limited, suggesting that institutional pressures dominate resource-based explanations during this period. Notably, disclosure gaps linked to emissions intensity narrow more quickly than those linked to absolute emissions, indicating that convergence occurs more rapidly for differences rooted in business model characteristics than for those driven by visibility and legitimacy pressures. Overall, the findings depict climate transition disclosure as an institutionalising field in which firms increasingly align with emerging norms but do so unevenly across disclosure dimensions and organisational contexts. The study highlights the distinction between the diffusion of reporting structures and the deeper development of transition management capabilities, contributing to an understanding of how sustainability disclosure practices evolve prior to regulatory mandates.