The negative impact of Invasive Alien Plant (IAP) species on natural areas, as well as other areas such as agricultural land, has been extensively researched. Certain mitigation strategies and programmes have been put in place such as the internationally recognized Working for Water Programme of the Department of Environmental Affairs. Such an initiative requires objectively determined spatial distribution data of IAP species at the required scale to allow for effective planning, implementation and future monitoring of IAP spatial changes. The National Invasive Alien Plant Survey project was initiated by the Working for Water Programme and implemented by the Agricultural Research Council. The aim was to establish and implement a cost effective, objective and statistically sound IAP monitoring system for South Africa, Lesotho and Swaziland at a quaternary catchment level. A complete inventory and a standard sampling approach both have limitations, mainly due to the size of the study area (127 million hectares) and variation in the natural environment, leading to high associated costs. An innovative sampling approach was therefore required. Phase I of the project dealt with the latter, whilst the current Phase II is a spatial refinement of Phase I.