Decreasing levels of student achievement in writing and the need for evidence about what drives New Zealand primary school teachers' practices provides the purpose for this research. This study investigates New Zealand upper primary school teachers' beliefs and influences related to writing instruction. A mixed methods exploratory sequential research design was utilised, and the first phase of data gathering consisted of eight case studies of eight different teachers of year 5-8 students. This provided the rich qualitative data required when exploring a topic with little research. Data gathering methods in this research included semi structured interviews, observations of writing practice and writing policies and planning documents. The case studies findings were analysed using the NVivo programme for constant comparative content analysis. These findings were then used to design the second phase of the study which was a nationwide online survey sent via Qualtrics to all schools with year 5-8 teachers. Descriptive statistics have been used to interpret the survey results and open-ended questions have been coded for content analysis. Both phases of data gathering have been used to discuss and answer the research questions. Findings from this research demonstrate that the influences on teachers practice are widely variable and complex, going well beyond the individual. Teachers primarily espoused constructivist philosophies about writing, and were highly focused on student engagement in writing, rather than writing achievement. Meaningful purpose and audience were considered essential to good writing practices and teachers employed a range of strategies to engage students in writing. Digital devices and tools for writing were used in a variety of ways although teachers also believed handwriting to be important. These findings raised questions of equity and time to teach a diverse range of transcription skills. Overall, teachers had difficulty articulating explicit links between classroom practice and both theory and curriculum. Writing was articulated as a complex skill to teach in comparison to other curriculum areas, partly due to the view of writing as subjective and personalised. Moreover, missing from the findings was a strong link to teacher-led, specific strategy instruction and content knowledge about writing. The strongest contextual factors influencing writing practice were identified as colleagues and resources, and the present research recommends using both these factors to influence teachers writing practice. These findings highlight the need for greater teacher support and structure in writing instruction driven from evidence of best practice research and theory.