Scenario: Flood and landslide — Nelson, Tasman and Marlborough

Understanding what could happen in a flooding and landslide event provides information on how to be prepared and think of ways to reduce risk.

Probability of similar rainfall in 50 years	Estimated damage costs	Estimated displaced residents	Exposed properties on Tāhunanui Slump	Duration of weather event
34%	>$50m	1000+	120	3 days

Probability of similar rainfall in 50 years: 34%
Estimated damage costs: >$50m
Estimated displaced residents: 1000+
Exposed properties on Tāhunanui Slump: 120
Duration of weather event: 3 days

What could happen?

This scenario explores what might happen if very heavy rain impacts the upper South Island for three days in a row. Rivers and streams in Nelson-Tasman and Marlborough would likely rise and overflow in some places, resulting in flooding of low-lying areas. Heavy rainfall could also trigger landslides in Nelson-Tasman and Marlborough, like the Tāhunanui Slump and other hilly areas.

This map shows the mapped extent of the Tāhunanui Slump area in Nelson. It does not show the level of risk for individual properties.

Map created from Slope Instability Susceptibility Area Nelson, Nelson City Council.

We can often forecast storms several days in advance, but we cannot predict exactly when or where the most severe impacts will occur.

An event like this has approximately 34% chance of occurring in 50 years. Based on science and previous events, like the 2022 storm event that affected the Nelson-Tasman and Marlborough regions, we know that storms could affect these regions again.

Research estimates that extensive land damage at the Tāhunanui Slump may be triggered every 10 to 50 years, with greater damage likely in the northern part of the slump.

Read more about storms, floods, and landslides and how they can affect people and places.

Flooding may cause water and debris to enter homes and buildings. Floodwater can damage floors, walls, and electrical systems. In some cases, floodwater can weaken the ground supporting a house.

If landslides are triggered, they may also damage homes and buildings, causing cracks in the ground, in building foundations, or bring debris onto properties.

In some cases, people may need to temporarily leave their homes to stay safe from floodwater or landslides. After flooding or landslides, properties will likely need to be checked by a qualified professional to make sure they are safe to use again.

Basic services may be disrupted for a while. Power cuts could last several hours, and phone, internet, water, and sanitation services may also be affected. In some places, people may need to boil water before drinking it. Roads might also be closed or blocked due to flooding or landslides.

By exploring the potential impacts and damage from flooding and landslides, we can understand ways to be prepared for these types of natural hazards. Councils and planners can use this information to help identify the most at-risk areas to guide decisions on building consents, land use, and emergency response plans.

For residents, it is important to know what natural hazards your property is exposed to and take steps to prepare:

Learn how to make your home safer from natural hazardsopen_in_new
Check that you have the right insurance in placeopen_in_new. A home insurance policy may qualify you for natural hazards cover.
Check your local council website for information about local hazards and emergency planning in your area.

Flooding in Richmond from the August 2022 severe rainfall event.

Photo: Tasman District Council

Landslide damage on a steep hillside in Nelson, New Zealand

A landslide on a hillside in Nelson. Landslides can damage land, roads and buildings, particularly on steep or unstable slopes.

Photo: Wendy Saunders