Background: Xanthorrhoea johnsonii A. T. Lee is a slow growing endemic Australian plant species. Its slow growth rate and lack of use by herbivores as a food source indicate that its leaves may contain chemical deterrents. Materials and Methods: X. johnsonii leaf extracts were tested for toxicity using the Artemia franciscana nauplii bioassay. Toxic extracts were further analysed by RP-HPLC and GC-MS. Results: As well as toxicity, anaesthetic/sedative effects were also noted in the methanolic and ethyl acetate extracts. A. franciscana nauplii 'died' following a 12 h exposure to these extracts, only to 'recover' and swim normally by 48 h exposure. By 60 h the X. johnsonii methanol/ethyl acetate extract exposed nauplii were again experiencing toxic effects and the mortality began to increase. The nauplii did not recover from this moribund state a second time. Analysis of these extracts by RP-HPLC showed that the ethyl acetate extract was the least complex of the toxic extracts. Subsequent analysis of this extract by GC-MS revealed that it contained 12 main compounds: o-hydroxycinnamic acid; acetic acid; 2,2-dimethoxybutane; pinacol; 4-methyl-1,3-dioxane; decane, 2,2-dimethylbutane; 2,5,9-trimethyldecane; 1,2-benzenediol; 1,2,3-benzenetriol; benzoic acid and an unidentified compound. Conclusion: These results confirm the toxicity and anaesthetic/sedative effects of X. johnsonii leaf components and highlight o-hydroxycinnamic acid as the likely compound responsible for this bioactivity.