With increasing environmental pressure due to global climate change, increases in global population and the need for sustainable obtained resources, water resources management is critical. In-situ sensors are fundamental to the management of water systems by providing early warning, forecasting and baseline data to stakeholders. To be fit-for-purpose, monitoring using in-situ sensors has to be carried out in a cost effective way and allow implementation at larger spatial scales. If networks of sensors are to become not only a reality but common place, it is necessary to produce reliable, inexpensive, rugged sensors integrated with data analytics. In this context, the aim of this project was to design and develop a low cost, robust and reliable optical sensor which capable of continuous measurement of chemical and physical parameters in aquatic environments. An iterative engineering design method cycling between sensor design, prototyping and testing was used for the realisation and optimisation of the sensor. The sensor can provide absorption, scatter, and fluorescence readings over a broad spectral range (280nm to 850nm) and temperature readings in real-time using a suite of optical sensors (CMOS Spectrometers and photodiode detector), custom designed LED array light source and a digital temperature probe. Custom electronics and firmware were developed to control the sensor and facilitate data transmission to an external network. Sensor electronics are housed in a marine grade watertight housing; the optical components are mounted inside a custom designed 3D-printed optical head which joins with the sensor housing. The sensor is capable of measuring a range of optical parameters and temperature in a single measurement cycle. Sensor analytical performance was demonstrated in the laboratory, for detection and quantification of turbidity using analytical standards and in the field by comparison with a commercially available multi- parameter probe (YSI, EXO 2). The laboratory and field trials demonstrate that the sensor is fit-for-purpose and an excellent tool for early warning monitoring by providing high frequency time-series data, operate unattended in-situ for extended periods of times and capture pollution events.