Evidence-based policy has been advocated by liberal democracies around the world. Through a comparative analysis of Australia and the UK, this research seeks to understand the development of evidence for adaptation policy by investigating policy players' perceptions of its usefulness and usability. In Australia, the evidence-based mandate has been weakened by prevailing politics, even though policy makers still seek to develop a business-case, for which climate science is often perceived to be incompatible. In the UK by contrast, evidence-based policy is enshrined in the Climate Change Act, yet how evidence has been developed under this mandate raises important questions about the extent to which it can ever be considered apolitical. Both cases reveal normative and political tensions in the development of policy evidence. Evidence lacks salience for policy players because it cannot fulfil the linear-technocratic promise of policy-making rhetoric. Evidence lacks legitimacy and credibility because it does not adequately account for varying local and contextual perspectives important for understanding adaptation problems; or, due to conflicting norms concerning what robust policy evidence is, and the subsequent priority given to a business-case versus the available evidence-base. Australia and the UK have developed adaptation-related policy through different means; although in both cases evidence development requires important normative choices by experts and bureaucrats before it is used (or rejected) in support of political goals. Policy evidence is developed, I argue, not just based on impartial technical analysis or under the potential influence of politics, but also according to the tacit norms of bureaucratic policy-making.