Energy Pulse NZ
Updated Jan 2026
On this page Current Hydro Thermal Outlook History

Risk Monitor

Live tracking of hydro storage, thermal availability, and system stress indicators.

The Risk Surface: Storage × Season × Probability

Dry-year risk isn't just about current lake levels — it's about when in the year those levels occur, and what the probability distribution of future inflows looks like. This matrix shows how risk escalates:

Storage Level
Jan-Mar
Summer
Apr-May
Autumn
Jun-Jul
Early Winter
Aug-Sep
Late Winter
Oct-Dec
Spring
>90th
percentile
Low<1%
Low<1%
Low<2%
Low<2%
Low<1%
50-90th
percentile
Low1-3%
Low2-5%
Moderate5-10%
Moderate5-10%
Low2-5%
25-50th
percentile
Moderate5-10%
Moderate10-15%
Elevated15-25%
Elevated15-25%
Moderate5-10%
10-25th
percentile
Elevated15-20%
Elevated20-30%
High30-45%
High35-50%
Elevated15-25%
<10th
percentile
High25-35%
High40-55%
Critical55-70%
Critical60-75%
High30-45%
How to read this: Percentiles compare current storage to historical levels for the same time of year (92+ years of data). A reading in the "10-25th percentile" in "Jun-Jul" means storage is lower than 75-90% of historical observations for that period — and there's a 30-45% probability of supply stress without intervention.

Risk probabilities are illustrative based on Transpower security of supply assessments and historical patterns. Actual probabilities depend on forecast inflows, thermal availability, and demand conditions.

Understanding Storage Levels

NZ's hydro storage is concentrated in a few key lakes. Here's how to interpret their levels:

🏔️ National Total ~3,200 GWh
Min (~1,500) Avg (~3,500) Max (~4,800)
Illustrative example: below average, within normal range
💧 South Island ~2,400 GWh
Min Avg (~2,600) Max
Pūkaki, Tekapo, Manapōuri, Te Anau — ~80% of national storage
🌊 North Island ~800 GWh
Min Avg (~750) Max
Taupō, Waikato scheme — smaller but less variable
Storage in context: National hydro storage of ~3,500 GWh sounds like a lot, but NZ uses ~120 GWh per day. That's roughly 30 days of supply from storage alone — much less in winter when demand is higher. This is why inflows matter as much as storage levels.

Key Lakes to Watch

Lake Operator Storage Role Key Characteristics
Pūkaki Meridian ~15% of national Largest single storage; feeds Waitaki scheme; snowmelt-dependent
Tekapo Genesis ~10% of national High altitude; consistent inflows; upper Waitaki
Manapōuri/Te Anau Meridian ~15% of national Rain-fed; powers Tiwai smelter; high inflow variability
Taupō Mercury ~8% of national Only NI significant storage; 1.4m operating range = ~1% of volume
Hawea/Wānaka Contact ~8% of national Feeds Clyde/Roxburgh; tourist area constraints

Storage percentages are approximate. See Transpower and Meridian for live data.

The Seasonal Risk Cycle

Dry-year risk follows a predictable seasonal pattern driven by inflows, demand, and when storage must be preserved:

Typical Annual Pattern

Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec

Bar height indicates relative risk level. Blue = normal operations, Amber = elevated concern, Red = critical period.

Jan-Mar
Lowest demand, building storage after snowmelt
Apr-May
Storage assessment window; decisions on thermal prep
Jun-Aug
Peak demand + lowest inflows = maximum risk
Sep-Dec
Snowmelt recovery; risk declining
The danger zone: The most critical period is entering winter (May-June) with below-average storage. At this point, there's limited time for inflows to recover, demand is rising, and generators must decide whether to conserve water or risk running dry in August.

Transpower's Risk Thresholds

The system operator uses formal risk curves to assess security of supply. These define probability thresholds that trigger industry action:

Threshold What It Means Industry Response
1% Risk Curve 1-in-100 chance of running out of stored energy Heightened monitoring; voluntary conservation; contingency prep
4% Risk Curve 1-in-25 chance of shortage Active demand response; industrial curtailment possible
8% Risk Curve ~1-in-12 chance of shortage Public conservation campaign; emergency protocols activated
How curves are calculated: Transpower runs 89 historical inflow scenarios forward from current storage levels, combined with demand forecasts and thermal availability. The percentage of scenarios that cross shortage thresholds determines the risk level. This is updated weekly.

Recent Risk Events

Source: Transpower Security of Supply assessments, Electricity Authority market reports

Scenario Comparison

How different storage and inflow combinations affect the system:

✅ Normal Year

Storage 50-90th percentile, inflows near average

  • Hydro provides ~55-60% of generation
  • Minimal thermal running except peaks
  • Wholesale prices ~$80-120/MWh average
  • No conservation required

⚠️ Dry Year Stress

Storage 10-25th percentile, inflows below average

  • Hydro reduced to ~45-50% of generation
  • Gas and coal running at higher capacity factors
  • Wholesale prices ~$150-250/MWh
  • Voluntary conservation; Tiwai demand response likely
  • Elevated risk of price spikes >$500/MWh

🚨 Shortage Risk Event

Storage <10th percentile, poor inflow outlook

  • Hydro conservation mode; sub-40% of generation
  • All available thermal dispatched
  • Wholesale prices >$300/MWh sustained
  • Public conservation campaign
  • Industrial curtailment possible
  • 1%+ probability of controlled load shedding
The good news: NZ has never actually run out of electricity. The system has always found ways to manage through — demand response, thermal fuel purchases, conservation campaigns. But the margins are tighter than many realise, and climate change may increase dry-year frequency.

What Determines Outcomes

Factor Better Outcome Worse Outcome
Inflows Early winter rain; good snowpack Extended dry; La Niña conditions
Thermal availability Gas supply secured; Huntly available Gas field outages; plant maintenance
Demand Mild winter; effective conservation Cold winter; high heating demand
Wind Good winter wind resource Calm, cold conditions (worst case)
Tiwai Demand response activated Full production maintained

Sources: Transpower, Electricity Authority, MBIE, industry analysis

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