Purpose - This paper aims to explore the nature of voluntary greenhouse gas (GHG) disclosure by non-GHG-registered companies among industry sectors over a period after the introduction of the National Greenhouse and Energy Reporting (NGER) Act 2007 and before the introduction of the Australian ETS. Design/methodology/approach - A GHG disclosure index is used to evaluate the levels of GHG disclosure in 2009 and 2011 annual reports. Findings - This paper highlights that non-GHG-registered companies seem to improve their disclosure by incorporating more "behavioural management" actions rather than "symbolic" actions. The changing rationale of GHG disclosure is towards more serious GHG reduction strategies. Consistent with voluntary disclosure and signalling theories, companies having good news to tell disclose their superior GHG information to promote their superior environmental performance. Research limitations/implications - The findings should be useful for stakeholders who are interested in GHG disclosure strategies. Also, the content analysis of the annual reports provides some clarity in respect of the most common aspects of GHG disclosure by non-GHG-registered companies which is helpful in the evaluation of correspondence between carbon disclosure strategies and the objectives of carbon abatement. Originality/value - Previous studies mostly investigate the differences in the type of GHG disclosure among companies subject to mandatory GHG regulations. However, this paper is the first study to examine the changing rationale in the nature of GHG disclosure of non-GHG-registered companies. While much of the prior research uses GHG-registered companies as the sample, no empirical study to date has considered non-GHG-registered companies that encompass 96 per cent of ASX listed companies.