The term 'Intermittently Closed and Open Lake or Lagoon (ICOLL)' has been adopted in NSW to described wave dominated barrier estuaries with an intermittent connection to the ocean. ICOLLs can also be found in south east Queensland, south-west Western Australia, and some parts of Victoria and Tasmania, although they are not the dominant estuary type as in NSW. From an international perspective, ICOLLs are also found in South Africa, New Zealand, Mexico and the Atlantic coast of Brazil and Uruguay. Within NSW, ICOLLs are mostly located south of Sydney, due to the high wave activity and close proximity of the Great Dividing Range to the coast, which results in small coastal catchments and thus small fluvial and sediment runoff. The distinguishing difference between ICOLLs and other estuary types is the variable condition of their entrances, which also makes them the most sensitive type of estuary to human interference (HRC, 2002; Boyd et al., 1992). The sensitivity of ICOLLs to external inputs has been described in this thesis based on their morphometric characteristics, which includes their size, shape and predominant entrance condition. NSW ICOLLs exhibit a wide range of physical conditions. Some ICOLLs are rarely open to the ocean, while others are rarely closed. Also, some ICOLLs have experienced extensive development within their catchments, while some are located mostly or wholly within National Parks and other protected reserves. When closed, ICOLLs behave like terminal lakes, retaining and assimilating 100% of the external inputs delivered to the system. When open, tidal flushing assists with advection and dispersion of inputs, however, significant tidal attenuation across the entrance still limits opportunities for effective removal of pollutants. The majority of NSW ICOLLs are considered to be mostly closed (i.e., have a closed entrance for more than 60% of the time), while remaining ICOLLs tend to be mostly open (i.e., have a closed entrance for less than 20% of the time). Few ICOLLs have entrances that are open and closed for roughly equal proportions of time, thus resulting in a distinctive bimodal behaviour of entrance condition (i.e., mostly open or mostly closed). NSW ICOLLs tend to be mostly closed unless (i) the catchment is larger than 100km2, and/or (ii) the exposure of the entrance to ocean swell waves is less than 60 degrees and/or (iii) the entrance channel contains geomorphic controls (e.g. shallow bedrock outcrops). Unless opened artificially, ICOLLs will generally remain closed until a sufficient volume of catchment runoff accumulates within the waterway to increase water levels to a level that overtops (breaches) the entrance sand berm. Once breached, high velocity flows over the berm cause scour and the development of a formalised entrance channel, which increases exponentially until an optimum width and depth has been reached (determined by the hydrostatic head, geomorphic controls and tidal conditions at the time). Following entrance breakout and lowering of the lagoon level, sand is reworked back into the entrance under the influence of flood tides and wave processes. The environmental condition of ICOLLs has generally been assumed as being dependent on the state of the catchment and the associated input of nutrients (form and magnitude) to the system. Biogeochemical processes also are reported to influence the condition of ICOLLs, particularly denitrification, which is controlled by the organic load on the bed and the extent of benthic algae and macrophytic productivity. In addition to this, however, it is demonstrated that the predominant and prevailing entrance conditions (i.e. open or closed) also influence the physical, chemical and biological environments. ICOLLs are particularly susceptible to the impacts of future climate change. This thesis provides a description of expected impacts on NSW ICOLLs environments associated in response to future climate changes, based on a detailed appreciation of physical processes and their follow-on consequences. Impacts on ICOLLs are expected as a result of increasing sea level, altered rainfall patterns, and modified offshore wave climate. A survey of relevant government officials has revealed that more than 50% of NSW ICOLLs are artificially opened before water levels reach the height of the natural entrance sand berm. Artificial entrance opening is mostly carried out to mitigate inundation of public and/or private assets around ICOLL foreshores, such as roads, backyards, farming lands and on-site sewage (septic) systems. Truncation of the hydraulic regime of ICOLLs can modify other physical, chemical and biological processes, and can result in deleterious impacts such as the terrestrialisation of estuarine wetlands and foreshores. Few statutory environmental planning mechanisms protect ICOLLs from future degradation. This thesis has identified the key issues that potentially compromise ICOLL integrity and sustainability, which include the expected future population growth in coastal NSW (thus increasing pressure for intensification of development within ICOLL catchments), future climate change (particularly increases in sea level), and the increased demand for amenity, particularly during summer holiday periods (i.e. 'summer impacts'). A series of management models have been developed to address key issues. The models comprise a suite of strategies that target future development and existing management practices, through a range of new or modified planning instruments. Models for the future management of ICOLL entrances aim to prevent artificial openings in the long-term. This requires, however, the systematic relocation, raising or flood-proofing of public and private assets that have been established on land that is potentially subject to inundation. Increasing sea levels in the future will compound the need for improved entrance management. Pro-active, integrated and adaptive management strategies need to be implemented today to minimise the on-going conflict and potential for continued environmental degradation in the future.