It is well recognized that protected area expansion targets in South Africa will not be met through traditional protected area models reliant solely on land purchase and rezoning by government. Rather, expansion of the conservation estate will also rely heavily on the contribution of conservation on private land. In this regard, a key existing policy instrument is the establishment of private nature reserves. However, private nature reserves present a particular policy design challenge around the harmonisation of incentive-driven and regulatory / command and control-driven requirements. In this paper, we apply a theory of change methodology to critically consider the challenges and opportunities for private nature reserves to contribute to the conservation estate in South Africa. The theory of change methodology is commonly used in policy evaluation to identify key assumptions and risks underpinning the internal logic of a particular policy intervention and/or mechanism. We, therefore, provide a critical evaluation of the regulatory context for private nature reserves, most recently supplemented through the publication of norms and standards in 2017. The results suggest that incentives promoting the expansion of private nature reserves need to be strengthened (such as income tax benefits), while the expanded regulatory framework risks dis-incentivising landowners to pursue private nature reserves on their land. We conclude by making a recommendation to advance and strengthen the potential of private nature reserves to contribute to the conservation estate in South Africa.