Background: Giardiasis is a debilitating disease caused by gastrointestinal parasites of the genus Giardia. Tasmannia lanceolata (Tasmanian pepper berry) has a high anti-oxidant capacity and has documented therapeutic properties for a variety of pathogenic diseases. Materials and methods: Solvent extracts of T. lanceolata berry and leaf were investigated for the ability to block G. duodenalis growth. The IC50 values of the extracts which displayed inhibitory activity were determined to quantify and compare their efficacies. Toxicity was determined using the Artemia franciscana nauplii bioassay. Active extracts were analysed by non-targeted HPLC-QTOF mass spectroscopy (with screening against 3 compound databases) for the identification and characterisation of individual components in crude plant extracts. Results: Methanolic, aqueous and ethyl acetate T. lanceolata berry and leaf extracts displayed potent G. duodenalis growth inhibitory activity. The methanolic extracts were the most potent growth inhibitors with IC50 values of approximately 180 µg/ml and 420 µg/ml for the berry and leaf methanolic extracts respectively. The aqueous, ethyl acetate, chloroform and hexane extracts also inhibited G. duodenalis growth, albeit with lower potency. HPLC-QTOF mass spectroscopy analysis of the extracts identified 45 compounds which were present in all T. lanceolata berry extracts. Forty of these were putatively identified by screening against 3 compound databases. All T. lanceolata berry and leafextracts were nontoxic in the Artemia fransiscana bioassay. Conclusion: The low toxicity of the T. lanceolata extracts and their potent G. duodenalis growth inhibitory bioactivity indicates their potential as medicinal agents in the treatment and prevention of this disease.