We investigated writing performance and knowledge of writing before and after a persuasive writing intervention for ethnically diverse elementary school students in a low socio-economic community in Aotearoa/New Zealand (NZ). We used a one-group pre-post design. The participants (n = 24) were year 5 and 6 students (ages 9-11) from the same classroom. Before the intervention, students completed writing tasks and a survey about their knowledge of writing. Next, they participated in a 5-week persuasive writing intervention in small groups. After the intervention, students again completed writing tasks and the survey. Results showed that after instruction, students' persuasive writing performance (d = 1.46) and knowledge of writing improved. These exploratory findings suggest that self-regulated strategy instruction is a promising approach for promoting writing outcomes for ethnically diverse elementary school students in Aotearoa/NZ.