Stormwater harvesting for residential and non-residential reuse is an important and crucial aspect to reduce freshwater demand to address climate change, population growth and urbanisation challenges. It is important that freshwater be conserved as much as possible through capturing rainwater and stormwater and using these resources for fit for purpose end uses such as irrigation of public open parks and residential non-potable end uses. The paper describes a methodology for the planning and design of a stormwater harvesting system for park irrigation. The application of suitable models for storage tank capacity and pipe sizing considering peak flows are described. The application of the approach is demonstrated with a local case study for the benefit of wider water professionals engaged in water-sensitive urban design.