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Salinity plays an important role in regulating the marine and estuarine ecosystem. In order to understand the effects of salinity in seawater and its influence on nearby estuaries, accurate, rapid, sensitive and selective methods are needed for sodium chloride determination. Therefore, this study offers a colorimetric assay method based on gold nanoparticles (average size of 7 nm) for determining sodium chloride concentrations in seawater and estuarine water. This provides a foundation for developing a reliable long-term salinity monitoring and sensing platform. The colorimetric assay from the interaction of gold nanoparticles with free sodium chloride in the water resulted in colour change which was detected by naked eye. Images of the colorimetric assay response were prudently analysed using the Colorgrab application and CIELab colour management in order to accurately fabricate the Lovibond colorimetric wheel for the quantification of sodium chloride in aqueous samples. The fabricated colorimetric device was applied to screen the levels of salinity along the coastal seawater and estuaries of Durban in KwaZulu-Natal. The results were comparable with those obtained using the traditional methods (ion chromatography and Mohr titration). Sodium chloride concentration in seawater ranged from 30 to 35 ppT. There was evidence of salt intrusion in Umgeni estuary with sodium chloride concentration decreasing to 5 ppT in a distance of approximately 400 m away from the sea. Overall, the developed AuNP colorimetric sensor proved to be an affordable tool, simple to use and provide analytical data that is accurate in a short time. [ABSTRACT FROM AUTHOR]