The RMA Quality Planning Resource

Mean high water and mean high water springs (MHWS)

Defining the position of MHWS is important as it is used to delineate the landward jurisdictional boundary of the coastal marine area (CMA) between the RMA and the Marine and Coastal Areas (Takutai Moana) Act 2011.

The definition for MHWS from New Zealand Almanac, Maritime Transport Division is:

The average of the levels of each pair of successive high waters during that period of about 24 hours in each semi lunation (approximately every 14 days) when the range of tides is greatest.

Further information on subdivisions with tidal boundaries and methods for determining MWHS is provided in the section of the guidance note on Defining boundaries next to waterbodies.

Movable water boundaries - key terms

Some key terms relating to water boundaries are defined in Appendix A of the BE Hayes publication: Elements of the Law on Movable Water Boundaries, March 2007:

Natural boundaries

Natural boundaries are not demarcated (that is, not marked with pegs or such) but are identified and delineated as to the tidal line or inland water line or middle thread that applies. This means that the land title register and plans which support it do not control the extent of the parcel on the ground where the rules of evidence of things there observed - the moving boundary - control the extent of the parcel at any given point in time.

Fixed boundaries

This term does not mean what it implies - boundaries that are guaranteed. What it does mean is boundaries that have been surveyed and geometrically defined and demarcated on the ground, and that the monuments (generally pegs) placed there may, if lost, be reproduced in accordance with the Survey Regulations. In relation to public access, the landward boundary of any road or other component of publicly owned margins under current interpretation is a fixed boundary. In contrast, the riparian (river) or littoral (sea) boundary of the publicly owned margins is almost always a moveable boundary. An express Crown grant of land surveyed by lines on all sides of the land including the water side may form an exception.


This is the gradual and imperceptible increase to land bordering water through the deposit of firm land on the banks of a river or stream, seashore or lakeshore, or through withdrawal of the water. Accretion may occur by (a) the washing up of sand or soil to form firm ground; or (b) the recession or withdrawal of water from the adjoining land as the result of seasonal changes in water level, or of longer term climatic changes such as drought conditions (a dereliction). When an accretion increases the width of a road, a reserve, or Crown land adjoining water, the increase in the parcel takes on the same legal character as the land to which it attaches. The "new" land is road if the land to which it attaches is road. Where there is no reservation of public land along water and there is a moveable natural boundary - that is, the water boundary is not defined by the lines of a survey to exclude accretion (which is rare) - there is an addition to the adjoining title. The moveable natural boundary may be tidal (the sea), an inland water line along a Crown-owned river, or a river or stream to which the presumption of ownership to the middle line applies.


Dereliction is an accretion of dry land gained by the gradual receding of waters.


Erosion is the loss of land adjoining water by the gradual and imperceptible action of the water. All riparian (riverside), littoral (seaside) and lakeside land may be subject to erosion. If privately owned to the water or the centre line, the land lost by the action of the water is lost to the registered proprietor; if a road or other public reservation lies between the land Crown granted and the water, the road or public land may be physically eroded away but (subject to individual assessment) will generally retain its legal status even if covered by water. If the advance of the water crosses the road or other reservation, the title of the registered proprietor will stay fixed at the Crown grant boundary, that is, the landward side of the road, reserved or other public land.


Diluvion is the slow advance of the waters over the land.


When the change of the position of the middle line of a river or stream has been sudden, violent and visible, as from the exceptional runoff from heavy rain or melting snow, the original middle line of the stream continues as the line of division of the two estates on the opposite banks of the stream. No question of accretion on the one side and erosion on the other arises.

The Ad Medium Filum Aquae Rule

By the common law, ownership of land adjoining a watercourse, which is not owned by the Crown, gives rise to the presumption that title extends to the middle line. It is a rebuttable presumption - that is, evidence to rebut (disprove) the presumption is always admissible.