September 2026 climate report

September has seen the start of spring conditions; there has been a lot of temperature variability with crisp mornings and warming afternoons. There have been dominant westerly flows bringing fronts to northwest parts of the region. The month has been drier than average especially in Eastern Coastal areas. The region is beginning to dry out with soil moisture below field capacity, groundwater has received less recharge.   

Summary

• Northland’s average rainfall was 62 mm, which was 52% of expected rainfall for September.
• September river flows for Northland range from “Below Normal” to “Extremely Low” across the region. Primarily “Below Normal” to “Low”. River flows are generally lower through Eastern, mid-north and southern parts of the region, there has been some recovery of Northwestern levels reflecting September’s rainfall distribution. 
• Lower rainfall totals this month have contributed to less groundwater recharge. Most levels are “Below normal” or “Low” for most of the region for this time of year. 
• Regional soil moisture is below normal for this time of year with most sites now below field capacity.
• El Niño is expected to peak in late spring or early summer, with the potential for this event to have significant impacts. The event is likely to rank among the strongest on record. The Relative Oceanic Niño Index (RONI) has already exceeded 1997 values for this time of year and has undergone the most rapid warming on record. 
• El Niño typically brings drier weather to Northland. 
• ESNZ’s Drought index is currently normal for Northland. 
• ESNZ’s Climate Outlook for October-December 2026 indicates that rainfall, soil moisture, and river flows are most likely to be below normal. 
• There is a moderate risk of drought conditions developing in Northland over the next three months. 
• The tropical cyclone season formally begins on 1 November.  

Rainfall

• Northland’s average rainfall was 62 mm across the region for September (Figure 1), which is 52% of the long-term median, or normal expected rainfall (Figure 2) for the month. Northwestern areas such as Waihopo, Hokianga and Waipoua had the most rain.
• The highest rainfall total for the month was recorded at Waimamaku at Weka Weka Road with 222.5 mm, 99% of normal expected.
• Other high recording rainfall stations include: Waima at Tutamoe with 174mm, 93%; Rotokakahi at Kohe Rd with 120 mm, 93%; Takahue at Saddle Road with 116.5mm, 73%; Tarawhataroa at Larmer Road with 99mm, 83%; Waihopo at Kimberley Road with 83mm, 120% of normal expected rainfall for the month.
• The lowest rainfall for August was recorded at Whangārei Harbour at Marsden Point with 19.5 mm, which is 25% of normal expected rainfall.

Figure 1 and 2: September 2026 rainfall distribution for Northland in mm, and in percentage of median. Showing an average of 62mm (52% of expected) rainfall for the region. 

Figure 3 and 4: Standardised rainfall for Northland on a 3-month period July to September 2026 and a 6-month period April to September 2026.

Both the 3-month and 6-month SPI maps show that the Northland Region ranges from Near Normal-Wet conditions to Extremely Dry in some areas. The last 3 months in particular have been dryer in a number of Central and Southern catchments. 

Figure 5: Rainfall accumulation graphs for the calendar year at key areas around Northland - January to September 2026.

• The six cumulative rainfall graphs above have been selected to provide an overview of the region, and the graphs show the historic min and max (bottom and top of the grey shaded area), the long-term median (dotted line), the years rainfall to date (red line) and last year’s rainfall (blue line).
• The graphs indicate that Northland as a whole experienced a wet summer and autumn, with many areas exceeding the historic cumulative maximum in April. Rainfall accumulation through winter has generally reduced and most areas are now closer to the long-term average for this time of year.
  

River flows

September river flows for Northland range from “Below Normal” to “Extremely Low” across the region. Primarily “Below Normal” to “Low”. River flows are generally lower through eastern, mid-north and southern parts of the region, there has been some significant recession of Eastern Coastal flows reflecting September’s lower rainfall distribution. 

Figure 6: River flows in Northland for September 2026

Groundwater

Lower rainfall totals this month have contributed to less groundwater recharge. Most levels are “Below Normal” or “Low” for most of the region for this time of year. The exceptions are Aupouri and Mangawhai where levels are “Above Normal”; Ruakaka and Coopers Beach are “Normal”; Awaroa where levels are “Very Low”.

Figure 7: Groundwater levels in Northland’s primary monitored aquifers for September 2026

Soil Moisture

Summary of modelled soil moisture data from Earth Sciences NZ

• The soil moisture deficit is calculated based on incoming daily rainfall (mm), outgoing daily potential evapotranspiration (mm), and a fixed available water capacity (the amount of water in the soil 'reservoir' that plants can use). 
• Soil moisture is below average with less rain in September and is below field capacity throughout Northland, except for across the border in Warkworth (Kaikohe AWS has had missing data during this period).

Figure 8: Calculated daily soil moisture values at key areas around Northland, courtesy of ESNZ, data ending 30 September 2026.

Figure 9: Soil moisture deficit in New Zealand as of 1st October 2026 courtesy of Earth Sciences New Zealand / NIWA

Figure 10: Soil moisture anomaly in New Zealand as of 1st October 2026 courtesy of Earth Sciences New Zealand / NIWA.

Climate Outlook for October – December 2026

Summary of climate forecasts from Earth Sciences NZ 

Seasonal Climate Outlook October - November 2026 | Earth Sciences New Zealand | NIWA

• Very strong El Niño conditions are now present in the tropical Pacific and are expected to strengthen further, according to criteria assessed by Earth Sciences New Zealand. El Niño’s influence on Northland’s weather patterns is becoming more apparent and is expected to be a dominant driver of climate patterns during this outlook period. 
• El Niño is expected to peak in late spring or early summer, with the potential for this event to have significant impacts. The event is likely to rank among the strongest on record. The relative Oceanic Niño Index (RONI) has already exceeded 1997 values for this time of year and has undergone the most rapid warming on record. 
• Late spring will provide above average temperature ranges; air flow patterns are expected to favour westerly, along with increased wind strength and gustiness. This pattern will bring more fronts to Western areas while increasing the potential for dry warm föhn conditions in eastern regions. Continued below normal rainfall in Northland is increasingly likely which will translate to below normal groundwater recharge creating challenges for water reliant sectors. 
• The tropical cyclone season formally begins on 1st November.
• Temperatures are likely to be above average (50% chance). 
• Rainfall totals are most likely to be below average (45% chance).
• Soil moisture levels and river flows are about equally likely to be below average (45% chance) or near average (40% chance)

Table 1: Forecasted likelihood of above, near or below average climate conditions for Northland from October – December 2026.

(Expressed as % likelihood).

Earth Sciences NZ Drought Index  

• The Drought Index is based on the Standardised Precipitation Index, the Soil Moisture Deficit, the Soil Moisture Deficit Anomaly, and the Potential Evapotranspiration Deficit, with a value less than 0.75 considered normal – wet.  
• The Earth Sciences NZ Drought Index for New Zealand indicates normal conditions in Taitokerau at the end of September (Figure 11). 
• Strengthening El Niño conditions and lower than average rainfall through Winter and the beginning of Spring have increased the likelihood of drought conditions developing later in the year. 

Figure 11: New Zealand Drought Index map as of 26th September 2026 courtesy of Earth Sciences New Zealand / NIWA.

Northland Drought Risk Assessment 

• The Drought Risk Assessment (Table 2) provides a visual summary of the current conditions of Northland’s rainfall, river flows, and groundwater levels and the current state of water resources. It also incorporates Earth Sciences NZ’s (ESNZ) Climate Outlook and Drought Index.  
• At the beginning of Spring, Northland’s rainfall, groundwater levels, river flows, and soil moisture levels are all below normal for this time of year. 
• ESNZ’s Climate Outlook for October-December 2026 indicates that rainfall, soil moisture, and river flows are most likely to be below normal. 
• There is a moderate risk of drought conditions developing in Northland over the next three months. 

Table 2: Drought Risk assessment matrix.