The development of low-cost environmental sensors is essential for effective management of our valuable water resources. Traditional water analysis involves manual collection of samples, transport and subsequent analysis in the laboratory. This is time and labour intensive, expensive and requires highly qualified personnel. Automation of this process enables more frequent analysis, saving time and money for researchers, industries and governing bodies. There is a huge and growing demand for low-cost water sensors as legislation becomes more stringent and as more frequent monitoring becomes essential for legislative compliance. The aim of this thesis was to develop low-cost colourimetric sensors for the determination of water quality in situ. Microfluidic technology was employed to facilitate miniaturisation of colourimetric analytical methods onto a portable sensing device, enabling mixing of small volumes of water with chemical reagents to form a coloured product in the presence of the analyte of interest. Two fully integrated centrifugal microfluidic sensors were designed and fabricated for rapid on-site water analysis. These sensors were developed for the determination of phosphate and chromium speciation in fresh water. Each sensor consisted of a microfluidic disc for method automation, a motor, absorbance based detection system, electronics board and was connected to a laptop for data collection. They were both validated against the standard colourimetric method, and applied to the measurement of river water and waste-water effluent samples. A fully autonomous phosphate sensor was designed, fabricated, validated and deployed in an agricultural catchment site. This sensor employed a microfluidic chip coupled with syringe pumps for fluid manipulation. It incorporated its own power supply and was programmed to take periodic unassisted measurements, with on-board data storage. Important considerations for sensor design included the sensitivity and selectivity of the analytical method, reagent stability, optical path length, material compatibility with reagents, use of low-cost components and overall robustness.