Venomous snakes are a valuable bio-resource for potential therapeutic drug design and development yet are also a major health hazard for society. Snakebite is a globally neglected tropical disease which disproportionately affects the poor and can cause debilitation and death. Snake antivenom is the only recommended treatment for snakebite, yet antivenom efficacy is often untested against many species for which it may be prescribed. In lieu of unethical testing in vivo, venom and antivenom tests in vitro can inform clinicians treating human and pet (domestic and livestock) snakebite victims. Australian venomous snakes (family: Elapidae; subfamily Hydrophiinae) are a specious group of some of the most venomous snakes in the world, whose bites can cause life-threatening coagulopathy (ie. disruption of blood haemostasis). This thesis focuses on Australian elapid venom effects on plasma-primarily human plasma-and the efficacy of currently available antivenoms. Chapter 1 introduces the main concepts and coagulotoxic theme and includes some redundancy with the introductions of individual chapters because all data chapters have either been published as papers or are prepared as such. For this same reason, knowledge gaps are identified within each chapter's introduction, rather than in Chapter 1. Chapter 2 focuses on the two most coagulotoxic genera within Elapidae: taipans (Oxyuranus) and brown snakes (Pseudonaja). The toxins (FXa:FVa) responsible for the exceptional coagulotoxicity of these venoms are a complex of snake venom Factor Xa, and the cofactor Factor Va. These toxins are homologous to mammalian prothrombinase complex-an enzyme complex which is central to the blood coagulation cascade. The efficacy of a newly developed antivenom was found to be surprisingly specific to the genus of snake in the immunising mixture (Oxyuranus) and largely ineffective against coagulotoxins within similarly related species (Pseudonaja). This likely renders the more affordable, new antivenom an unsuitable solution for snakebite by the eastern brown snake (Pseudonaja textilis) in Papua New Guinea. Chapter 3 broadens the scope of the thesis to include an unprecedented sample size of 47 Australian venoms across 42 species and 19 genera, investigating Australian elapid venom effects on human plasma. This timely research uncovered an additional four genera that contain species with previously unknown potent procoagulant venom. Tiger Snake Antivenom was broadly cross-reactive but varied extensively in relative efficacy across venoms. This chapter also examines the molecular evolution of the prominent coagulotoxin within these venoms-Factor Xa-which revealed a clear, stepwise evolution of increased cleavage region and potency via diversifying selection. Chapter 4 investigates the anticoagulant properties of all currently recognised species of black snakes (Pseudechis) and nine localities of mulga snake (P. australis)-the most widely dispersed and Publications included in this thesis Incorporated as Chapter 2, 'Browns/Taipans', published