Maketu Estuary is situated -35 km southeast of Tauranga in the central Bay of Plenty, New Zealand. The hydrodynamics of the estuary was drastically altered when the large Kaituna River was diverted directly to the sea in 1957 as a part of the flood protection scheme. This dramatically changed the inlet hydrodynamics, caused deleterious effects on sedimentation and navigation, and catastrophic effects on the large area of wetlands on the estuary margins. This thesis represents study of the relative importance of physical processes (hydrodynamic and sedimentary) which govern the overall physical behaviour of Maketu Estuary in response to the 1996 partial re-diversion of the Kaituna River back into the estuary. A detailed hydrographic survey of the entire Maketu Estuary was undertaken by combining two survey methods (land and boat-based) and this study showed the estuary to consist of large inter-tidal areas covering 70-80% of its -2 km² area. A bathymetry map of the estuary was produced showing that Maketu is a small geomorphologicaly "perched" and "low meso-tidal" estuarine lagoon dissected by numerous narrow tidal channels, 10-30 m wide with typical maximum depth of 1 m below MSL. At present, the inlet entrance is about 120 m wide and 2.6 m deep at high tide. A bottom-towed ADP technique was used to collect velocity profiles in the narrow and shallow inlet throat. The data were analysed for entrance inflow and outflow calculations. The freshwater inflow from the Kaituna River through the floodgate control structure reduced the flood tidal volume by 34% and increased the ebb tidal outflow by 48.3%. The calculated change of the inter-tidal storage between 1985 and 1996 suggests a net sedimentation in the estuary of about 150,000 m³ or 13,640 m³/ year. Stability parametres of the inlet gorge were re-examined and implied that the inlet was generally stable, but with a tendency for scouring of the inlet channel during the ebb tide following the rivers partial re-diversion. In its present form, the inlet lies within the stable region of the stability/closure curve with entrance closure likely to occur if the tidal prism decreases below 255,000 m³. A Ω/Mₜₒₜ ratio of - 25 classifies Maketu inlet as a "barbypasser" having positional stability, with an estimated increase of the ebb shoal volume of - 17% in comparison to 1985. Three tide gauge stations were established within the estuary to provide long-tidal records (from September 1995 to June 1996) covering periods before and after the partial re-diversion of the Kaituna River back into the estuary. Analyses revealed that amplitude decay of the M₂ tide constituent from the ocean (- 0.74 m) to the far reaches of the estuary is approximately 68%, while the M₄ principal overtide within the estuary is > 0.07 m. Forced MSf tide is the largest compound constituent (>0.06m) while the frictional nature of the estuary is demonstrated by large phase changes in the tide (- 117° in 3 km for the M₂ constituent) and a M₄/M₂ ratio of 0.31. The amplitude ratio M₄/M₂ demostrated a slower growth with distance (lower to upper estuary) after river re-diversion, and the tidal duration asymmetry parameter diminished by 6.4%. Non-tidal influences on the sealevel oscillations within the estuary (caused by attendant weather systems during the observation period), showed that the set-up or set-down of the sea level ranged around ± 0.2 m. Seiches were identified in the lower estuary possessing a fluctuating range of 0.12 m with mean period of - 20 minutes. The longer flood duration and associated reduction in current strength at the upper reaches of the estuary should lead to reduced sedimentation within it after the partial re-diversion of the Kaituna River. Current dynamics of Maketu Estuary is dominated by the presence of strong tidal currents and intermittent river inflow within the estuarine channels (mid-flood and mid-ebb subsurface currents of 0.94 and 1.27 ms⁻¹ respectively). Harmonic analysis showed semi-diurnal current regime with tidal current (M₂) ranging from 0.15 ms⁻¹ (at a site in the upper reaches of the estuary) to 0.74 ms⁻¹ (near the open entrance). The analysis revealed clockwise rotation of the M₂ tidal currents at the upper estuary, which lagged the anti-clockwise rotating M₂ tidal currents of the lower estuary by 22⁰. The M4 tidal current has a large amplitude (> 0.05 ms⁻¹) throughout the estuary indicating a significant transfer of energy from the M₂ current caused by friction at the seabed. The tidal rectification caused by the intermittent river inflow after partial re-diversion generated a significant residual current regime. Residual along-channel current gradually increased up-harbour and expressed as a percentage of the tidal signal it was 4%, 16% and 20% respectively. Meteorological forcing plays a significant role in the circulation dynamics of the Maketu water masses. At times, the current flow was directed upwind in the deeper channels. Analysis of the long-term current measurements revealed a strong coherence between low-frequency wind and low-frequency current signal at all observed sites. A rotary spectrum for the mean residual flows indicates that significant peaks occur within a period of 3.7 days between wind and current. Different vertical velocity structure over the whole water column was depicted at observed sites indicating an altered flow structure due to the presence of a stratified water column. The thermohaline structures and associated physical processes within the water have been investigated in detail for the periods before and after the partial river re-diversion. According to the "classical" tidal prism model, the residence time was estimated to be 18.2 hours for the lower estuary and 30.8 hours for the upper estuary. After the 1996 partial re-diversion of the Kaituna River, the salinity-temperature structures of the estuarine waters have been changed, promoting strong conditions dependent on the specific operating regime of the control floodgates. Analysis of salinity-temperature long-term measurements at fixed sites within the estuary revealed a large variability in salinity over the course of the tidal cycle at the surface and bottom (24-30 psu). The major influence on the salinity stratification is due to river inflow (-33%) and wind stress (-21 % ) associated with prevailing south-westerly wind blowing across the estuary. The residence time is approximately a half-tidal cycle shorter than before the partial river re-diversion. The turbulence and mixing processes which characterised water mass behaviour is a combination of small-scale turbulent diffusion and a larger-scale variation of the field of advective mean velocities. The components of Reynolds stress parallel to the boundary in the direction of mean flow were found to be variable in the range of 0-12 cm²s⁻² depending on tidal phase, while the energy spectra of turbulent velocity fluctuations were found to be similar (both during the flood and ebb phase) which indicates that the weak turbulence is associated with a weak mean current and stratified water column. The Brunt-Vaisala frequency and the local gradient Richardson number indicate dynamically stable ebb flow (Rᵢ >12), but during the flood phase the vertical distribution of the Richardson number (Rᵢ ≤1) indicates instability of the flow, likely associated with the river inflow (a fully turbulent mid-flood flow). The Ozmidov scale varies from 0.7-1.8 m and is comparable to the largest scale of the flow at the sites examined, while the smallest scale, the Kolmogorov scale was estimated to be - 3 x 10⁻² cm. An axial convergence front was formed only during the late stage of the flood tide extending continuously up the main estuary channel, while occurrence of internal waves was identified in the upper reaches of the estuary. Derived typical values of Nz (vertical eddy viscosity) ranged from 8.3-23.8 x 10⁻⁴ m²s⁻¹, Kz (vertical eddy diffusion) ranged from 4.4-9.2 x 10⁻⁴ m²s⁻¹ and Kₓ (longitudinal eddy diffusion) ranged from 1.24-8.3 m² s⁻¹. Overall, vertical density gradients caused by the presence of the newly-formed salinity-temperature regime had a stabilising effect on the vertical mixing processes within the water column due to the re-diverted river. As a complement to the field observation, a two-dimensional depth-averaged numerical hydrodynamic model (3DD model of Black's operating in 2D mode) for the tidal and current propagation within the estuaries was employed to obtain a better understanding of the complex Maketu Estuary water mass dynamics. Summarising, the 2-dimensional hydrodynamic (2DH) model for Maketu Estuary was shown to have the capacity to be used (i) as a useful diagnostic tool in environmental management of the Maketu estuarine system; and (ii) in planning future field sampling and investigating dynamic flow responses to changing forcing variables, if used cautiously in stratified conditions.