Weather Events June-July 2025 Background Information
In late June and early July Marlborough experienced significant rain events resulting in the declaration of a district-wide State of Local Emergency at 3.30pm on Friday 27 June.
The rain brought widespread flooding to the Te Hoiere/Pelorus, Kaituna, Tuamarina, Wairau, Taylor and the Awatere River catchments including extensive surface flooding impacting communities, public assets and private land.
Preliminary rainfall totals for the event were more than 250mm in the Rai/Pelorus catchments, more than 250mm for the Richmond Ranges (Wairau, Northbank), and around 120mm for the Blenheim urban area.
The weather events generated record high groundwater levels across the eastern Wairau Plain, Rārangi, Riverlands, Southern Valleys, Northbank and Wairau Valley. It also put Blenheim’s stormwater and sewage network under strain.
Marlborough Emergency Management gave early warning to exposed communities in Spring Creek and Renwick, with parts of Spring Creek evacuated as a precautionary measure.
Groundwater Flooding
Background
High groundwater tables because of June-July weather events have affected many of the lower lying areas of the Wairau Plain including Rārangi, Spring Creek, Renwick’s Lower Terraces and Wairau Valley.
June was the wettest recorded in Blenheim since 1942. The rain event from 26 to 28 June brought widespread flooding to the Tuamarina, Wairau and Taylor River catchments, as well as the Te Hoiere/Pelorus, Kaituna and Awatere River catchments. This included extensive surface flooding, impacting communities, public assets and private land.
The heavy rainfall resulted in an extremely high groundwater table and soil saturation across the eastern Wairau Plain, Rārangi, Riverlands, Southern Valleys, Northbank and Wairau Valley. It also put Blenheim’s stormwater and sewage network under strain.
Groundwater
What is groundwater?
Groundwater is water that exists beneath the Earth's surface, filling the spaces within soil, rocks, and sediments. It is a crucial component of the water cycle and a vital source of freshwater for many communities. Unlike a stream, groundwater is stationary with no discernible water movement.
How is groundwater level determined?
Groundwater levels are determined by the balance between rainfall, surface water infiltration and the underlying geology.
What is groundwater flooding?
Groundwater flooding can happen during periods of unusually high rainfall, when the rising water table causes water to rise out of the ground. Groundwater flooding can bubble up and start flowing along the surface and can also rise directly in homes.
Why have groundwater levels increased in some areas?
In areas with permeable soils and high rainfall, water naturally accumulates underground. In coastal or low-lying areas, rising sea levels and more intense rainfall are pushing these groundwater levels higher.
What can be done about high groundwater levels?
Groundwater will recede slowly as the water tables drop so it is a matter of time – usually weeks rather than days, and in the absence of more rainfall, to return to normal winter levels. While the surface drains and pumps can help manage water temporarily, these measures do not lower the water table permanently. In fact, if the water table is high enough, drains can become ineffective or even backflow during heavy rain.
Are there any other solutions?
Large-scale solutions like deep wells, sump systems or subsurface draining networks are costly, require constant maintenance and can cause unintended consequences like drying out wetlands or affect neighbouring properties.
What is Council doing about this issue?
Council has carried out some preliminary investigations and found that there would be only a marginal improvement in water ponding levels in the Rārangi area, even with a very significant capital expenditure investment. A high groundwater table causes surface water to run straight off the land and means there is nowhere to pump the water to. This issue requires collaboration across Council and a staff working group will be set up to consider it further for the future.
Water Quality
What does groundwater flooding mean for water quality?
Residue floodwaters, either from groundwater flooding or surface flooding, can be contaminated with germs which can make people unwell. When groundwater breaks the surface, there is potential for the mixing of drinking water from shallow wells with human effluent from onsite wastewater treatment systems (septic tanks). This can contaminate drinking water, land and any surfaces it touches.
Should I boil my water?
It is important that water used for drinking, preparing food or drinks (including baby’s formula), brushing teeth, or washing dishes and utensils is safe and free from contamination that could cause illness. Bore water owners should consider boiling their water as a precaution in areas where floodwaters or groundwater flooding is present. This applies if you do not have your own maintained treatment system on your supply such as a 5-micron (or less) cartridge filter followed by an ultraviolet disinfection unit.
What is best practice?
To boil water for use for the above you should bring water to a rolling boil for at least one minute, then allow to cool naturally before use. You can refrigerate until needed. Any boil water advisory includes water used for the following:
- Drinking
- Food Preparation
- Brushing teeth
- Making ice or juices
- Preparing baby formula
- Pet drinking water
Rārangi
Some parts of Rārangi have been particularly affected by high groundwater levels.
The groundwater flooding issue here has only recently become apparent as, prior to 2022, groundwater levels in the Rārangi Aquifer had never reached these levels (going back to the first measurements in 1989). In July 2022, after a similar major rain event, it took about two weeks for the surface water on Rārangi Beach Road to subside.
These are quite rare events based on feedback from residents and since the late 1980s may only have occurred to this extent in 2022 and 2025.
Intense rainfall fell at Rārangi and in the western ranges over three days from 26 June 2025. The rainfall monitoring site in Rārangi at Driving Range recorded 216 mm of rainfall from 25 to 29 June. During this storm, the highest 6hr, 12hr, 24hr, 48hr, and 72hr rainfall totals were recorded since the Driving Range site was established in 2010.
This intense rainfall rapidly filled the Rārangi Shallow Aquifer, with record high levels observed across all Council’s inland monitoring wells. With the Rārangi Shallow Aquifer full it broke the surface, expanding the extent of the wetlands to capacity, before generating significant runoff southwards to the Wairau Diversion mostly via the Hinepango wetland.
In this state groundwater temporarily breaks the surface in low lying areas between the dunes laid down by the old coastline. This caused issues for buildings, roads and infrastructure, essentially a ‘groundwater flood’. Because of the flow rate, it could not be pumped away or drained like surface flooding.
If drainage did occur, which would require a significant investment in infrastructure, it would cause issues in droughts for domestic water supplies in the area, as well as having an ecological effect on the wetlands.
The Rārangi Shallow Aquifer drains slower than nearby rivers as there is more resistance as the water moves through the coarse sands and fine gravels of the aquifer.
Further significant rainfall on 3 and 4 July (48.5 mm) and again on 11 July (43 mm) kept groundwater levels high, with limited opportunity for water to drain over the 14 days since the initial event.
The Rārangi Shallow Aquifer groundwater broke the surface on 28 June 2025 with levels taking a month to recede back below the surface.
The total rainfall from 25 June to 11 July 2025 was 308 mm. Average annual rainfall at this site is 950 mm, so the rainfall during this 16-day period was equivalent to about 4 months’ worth of normal rainfall.
In comparison, over the August 2022 storm event, 140.9 mm of rain was recorded from 16 to 20 August, however groundwater conditions prior were wetter in 2022.
