Didymosphenia geminata is a diatom that has been accidentally introduced to New Zealand's South Island rivers. It has grown to bloom conditions in all rivers it inhabits, which has caused impacts to the river systems, loss in recreation value, and economic losses. The pathways and vectors of dispersal are difficult to control and hence it continues to spread throughout the South Island. Laboratory experiments assessed the survivability of D. geminata in different environmental conditions, a range of combinations of light availability, temperature and moisture. Experiments in the field were based in the Waitaki River to determine growth rates of D. geminata. D. geminata is growing in a greater range of temperature and light conditions than previously recognised. In cool to cold conditions with a little water this diatom can survive up to 1500h, the colder temperatures also increase survivability in the dark. However, D. geminata has reduced survivability in warm, damp conditions, up to 60h. In the Waitaki River D. geminata is attaining biomass of 2.51mg mm-2 over six weeks during summer. This high biomass is causing a change in biotic and abiotic conditions. Longevity of survival and the range of conditions in which it can survive increases the risk of spread throughout New Zealand and the world. There are considerable problems with invasive species and international trade. Policies aiming to reduce international invasions due to trade are becoming more prevalent as the consequences of invasion are more obvious and costly. New Zealand has been able to implement polices in the last decade that has reduced the number and variety of incursions. However these policies did not stop D. geminata arriving. This shows that even with the best policies species can invade fragile ecosystems. Central and local government policies surrounding management of invasive aquatic species were reviewed. Central and local government policies were developed to reduce the spread of D. geminata, however they are not effective as the diatom is still dispersing. Further research is required to elucidate means of dispersal in New Zealand, in particular the importance of dispersal by animals compared with that by humans, and the importance of continuing international dispersal.