The spatial and temporal variability of dissolved reactive silica (DSi) and salinity in Tairua Harbour (Coromandel, New Zealand) were investigated along with that of the riverine inputs and adjacent coastal waters. In all surveys, covering a range of fresh water discharges and seasons, the very high linear correlations between DSi concentration and salinity indicated conservative behaviour, with physical dilution being the only process having any effect on the distribution of DSi in both estuarine and coastal waters. The tidal exchange of DSi, and hence estuarine water, with the coastal water body was high, with around 82% of the incoming flood tide comprising “new” ocean water. This exchange process was strongly influenced by the quasi‐periodic non‐tidal alongshore current, but only weakly by variation in river discharge and tidal range. It appears that complete mixing of fresh water (under normal flow conditions) and riverine solutes occurs over a relatively narrow coastal zone (2–3 km) along this stretch of the Coromandel Peninsula.