Average sea level rise measured in Lyttelton, New Zealand

This indicator measures the rise in annual mean coastal sea level relative to land. The national mean is derived from four long-term monitoring locations across New Zealand: Auckland, Wellington, Dunedin and Lyttelton. Relative sea-level rise includes the vertical land movement of the surrounding area (for example, a sinking landmass increases the rise in ocean sea level).
Land Information New Zealand reports the change in annual mean coastal sea level to 2018 against the established baseline (mean sea level for 1986–2005) for the long-term sites plus an additional two sites: Moturiki (Mount Maunganui) and New Plymouth. These are not included in the national mean due to shorter records.
Coastal sea level rise has been calculated from 1901, when the last of the monitoring sites was installed.

https://www.stats.govt.nz/indicators/coastal-sea-level-rise

Data were not available for all years for Auckland, Wellington, Lyttelton, and Dunedin, but cumulatively these records cover at least 100 years. While the Moturiki and New Plymouth annual mean sea-level time series cover a shorter period, 46 years and 65 years respectively. These datasets were augmented by a short-term mean sea level over a few years for an earlier period of measurements at both sites with data available from Land Information NZ archives. These earlier mean sea level values were used to set the zero of local survey datums: Moturiki Vertical Datum 1953 and Taranaki Vertical Datum 1970. Previous analyses have used an inferred value for mean sea- level rise in New Plymouth and Moturiki to account for the short time series. For this analysis, the non-inferred value for New Plymouth is used because as more data is available and as the time series lengthens, there is more confidence in the non-inferred estimate of sea-level rise. Because of the shorter time series in the Moturiki dataset, an inferred value is still used.

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Relative sea-level rise, along with future projections, is what local communities need to address when adapting to rising sea level. Rising sea level affects our coastal communities, infrastructure, environments, and biodiversity. Communities (including marae) and infrastructure (including urupā) are directly affected by rising sea levels through increased risks of flooding and erosion. Rising sea levels will also affect biodiversity, taonga species, and coastal habitats. Estuaries, coastal wetlands, and coastal margins we inhabit will be at risk of being lost due to coastal squeeze. This occurs when natural habitats are eroded or flooded by the sea and cannot retreat due to arable, built, or unsuitable land on their landward edge.

Natural hazard events that were once very rare will become more common. For example, over the past century rising sea levels have led to an approximate doubling in the number of days when tides reached the 1900 ‘king tide’ level. In New Zealand, this level is used as a ‘red-alert’ to warn of increased potential for coastal inundation. Drainage of stormwater systems in low-lying areas that drain naturally by gravity will be increasingly difficult to drain as flooding, wave overtopping, and groundwater levels increase.