Over the last few decades, due to increasing global awareness of environmental issues, there has been great interest and motivation in research to develop natural fibre composites to replace glass fibre composites in certain applications. Harakeke fibre known as New Zealand flax or Phormium tenax used to be an important export material in New Zealand in the early twentieth century, but its production reduced due to the availability of synthetic fibres midcentury and competition from other natural fibres such as sisal and flax. Now, harakeke plants are planted mainly for landscaping with some fibre extracted from harakeke leaves used for craft goods and traditional products of Maori people who were the earliest settlers in New Zealand. Only two family workshops in the country are currently extracting harakeke fibre for this purpose. The aim of this thesis was to assess if harakeke fibre has potential for reinforcement in polymer composites and to assess the hybridisation of harakeke/hemp hybrid composites. Both short and long fibres were used as reinforcement, while a low viscosity epoxy resin was used as the matrix. Short fibres were alkali treated before being used to reinforce epoxy, while long fibres were used as supplied. Short and long fibres were aligned using dynamic sheet forming and manual carding, respectively. Composites were produced using hand lay-up and compression moulding. The physical and mechanical properties of fibres and composites were tested following ASTM and ISO standards. The surfaces of fibres and fracture surfaces of composites were assessed microscopically using optical microscopes and scanning electron microscope (SEM). Short harakeke and hemp fibres were alkali treated at elevated temperatures in a fibre pulping digester with a solution of 2wt% NaOH or 5wt% NaOH/2wt% Na2SO3. Single fibre tensile testing was carried out on untreated and treated fibres. It was found that harakeke fibre treated with NaOH and hemp fibre treated with NaOH/Na2SO3 retained their tensile strength and Young’s modulus compared to the untreated, while harakeke fibre treated with NaOH/Na2SO3 was degraded. Fibre surfaces and fibre separation were evaluated revealing that fibres treated with NaOH/Na2SO3 had better separation and rougher surfaces compared to those treated with NaOH. Densities of harakeke and hemp were found to increase after alkali treatment. Fibre lumens were found to make up significant volume of fibres with 41% of single fibre volume and 21% of fibre bundle volume for harakeke fibre and 18% and 11% for hemp fibre, respectively. Lumens were found to be a major factor contributing to porosity of long aligned harakeke composites. Mechanical properties including tensile, flexural and fracture toughness of composites containing aligned short 2%NaOH treated harakeke fibre with different fibre contents were evaluated and compared with randomly oriented harakeke/epoxy composites. It was found that all properties increased with fibre content. Tensile strength and Young’s modulus of the aligned short fibre composites at the optimum fibre content of 46wt% were 136 MPa and 10.5 GPa, respectively. These values are higher than any reported in the literature to date for natural fibre composites excluding those where hand-layup or a continuous fibre form has been produced and furthermore, these values overlap with those achieved using these procedures. Mechanical properties of aligned long harakeke composites were also evaluated. The tensile strength and Young’s modulus were comparable to those for sisal and hemp composites in the literature and their specific values were comparable to those for glass fibre composites. Impact strength and fracture toughness of harakeke fibre composites that have not been seen previously in the literature were found to be 132 KJ/m2 and 7.69 MPa.m-1/2, respectively, at a fibre content of 63wt% for aligned long harakeke/epoxy composites. These values are higher than any reported in the literature to date for natural fibre polymer composites. A Rule of Mixtures based model was developed for predicting aligned long harakeke fibre composite strength with the assumption that composites fail when fibres with the lowest failure strains failed and considering the effect of porosity. It was found that porosity affected tensile strength as well as Young’s modulus of the composites. Aligned long or short harakeke/hemp hybrid biocomposites were prepared with different fibre lay-up and weight ratios between harakeke and hemp and their mechanical properties were assessed. While tensile properties, impact strength and fracture toughness (KIC) of the harakeke/hemp hybrid biocomposites were found to be independent on fibre lay-up, flexural properties were found to be dependent as would be expected due to the influence of second moment of area. The fibre failure strain based hybrid effect (FS hybrid effect) defined as the enhancement of low elongation fibre due to presence of high elongation fibre in composite and the Rule of Mixtures based hybrid effect (ROM hybrid effect) termed as the deviation of a certain property from the Rule of Mixtures were assessed for harakeke/hemp hybrid biocomposites. While the FS hybrid effect was found not to be observed, ROM positive hybrid effects were found to be observed for fracture toughness of aligned short harakeke/hemp hybrid biocomposites with different fibre lay-ups and different relative fibre contents. ROM positive hybrid effects were also found to be observed for flexural modulus for comingled harakeke/hemp fibre composites at different relative fibre contents. Tensile properties and impact strength were found to obey the Rule of Mixtures.