Extreme weather - heavy rainfall

Heavy rainfall is one of the most frequent and widespread severe weather hazards to affect New Zealand.

Heavy rainfall is one of the most frequent and widespread severe weather hazards to affect New Zealand. It is defined as rainfall greater than 100 mm in 24 hours.

In New Zealand, heavy rainfall is relatively common. Often, a significant amount of precipitation occurs in only a few hours, leading to severe flooding and landslide risk.

Causes of heavy rainfall

Heavy rainfall occurs over New Zealand mainly because of the following common weather systems:

  • ex-tropical cyclones
  • North Tasman Sea lows moving to NZ region
  • depression/lows from the south
  • cold fronts.

New Zealand's mountains tend to modify and amplify precipitation, and this often causes the frequent heavy rainfall we experience. Heavy rainfall tends be most common over the western coastal region of the South Island and the middle and upper North Island, and least common on the east side of the South Island (due to the prevailing westerlies).

Potential consequences of heavy rainfall

Heavy rainfall can lead to numerous hazards, for example:

  • flooding, including risk to human life, damage to buildings and infrastructure, and loss of crops and livestock
  • landslides, which can threaten human life, disrupt transport and communications, and cause damage to buildings and infrastructure.

Where heavy rainfall occurs with high winds, risk to forestry crops is high.

Flood debris on the Kerikeri Basin bridge during the flooding of March 2007. Photo: FNDC.
Large rock and debris slide in the Manawatu Gorge which blocked SH 3 for about 3 months after initial failure during the February 2004 heavy rainfall.
Left: Photo-like image from MODIS on NASA’s Aqua satellite on 13 June 2006 showing the snow cover over the South Island, New Zealand from a winter storm swept New Zealand on June 12. The snow stretches to the sea on the east side of the island. Right: Satellite imagery from MODIS Aqua at 0159 NZST 16 February showing the deep clouds associated with the deep depression that brought heavy rainfall to the lower and middle North Island


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    Flooding - how does it happen?

    Education Resource
    Understanding what happens above and below ground during a flooding event, and how it is different from normal rainfall conditions