Security of Supply and ERCs

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Latest Weekly Update Reported on 14 September 2021

(based on data at midnight 12 September)

Current Storage Positions

National hydro storage increased this week to 125% of average for the time of year (76% of full). This was the result of above average inflows in the South Island.

North Island storage increased three percentage points to 83% of the historical average. South Island storage increased by 11 percentage points to 131% of the historical average.

Market Indicators

Weekly Prices

The average price at Haywards was $93/MWh, up 45% from $64/MWh last week. 

Prices at Haywards peaked at $493/MWh on Wednesday morning, corresponding with high demand (the first evening of Alert Level 2 outside of Auckland) and moderate levels of wind generation.

This Friday inter-island price separation and high North island prices followed the unplanned outage of Pole 2 of the HVDC system from 07:30 to 13:00. The Pole 2 outage is believed to be related to very high winds driving the conductor too close to a tower located in the northern Canterbury region.

On Sunday there was intra-island price separation in the lower South Island. Transmission was constrained due to the potential loss of both the Clyde – Cromwell - Twizel circuits 1 and 2 being managed as a single event, due to a severe te uira (lightning) storm in the area.

Weekly Demand

National weekly demand increased 23 GWh (three percent) on the previous week to 785 GWh. 

This week's peak was 6,492 MW and occurred at 18:30 on Monday 8 September 2021. 

Generation Mix

Total generation was 821 GWh this week, with hydro making up 60%, and thermal 8% of the mix. Wind generation was high at 79 GWh, or 9.6% of the mix; up by 27 GWh from last week.


HVDC transfer has remained Northward throughout the week. HVDC transfer exceeded a 1,000 MW on Saturday evening.

Electricity Risk Curves

New Zealand and South Island storage are both in the Normal range.

Electricity Risk Curves

New Zealand controlled storage is above average and South Island controlled storage is above average. The graphs below compare New Zealand and South Island controlled storage to the relevant Electricity Risk Status Curves.

The graphs below compare New Zealand and South Island controlled storage to the relevant Electricity Risk Curves - Percentage Risk.

Click here to learn more about the Electricity Risk Curves and thermal fuels validation

Electricity Risk Curve Files
Latest New Zealand Electricity Risk and South Island Electricity Risk Curves [ pdf 626.68 KB ]
Electricity Risk Curve Data [ xlsx 48.38 KB ] (Effective from 23 August 2021)
Simulated Storage Trajectories Files

Simulated Storage Trajectories [ pdf 300.28 KB ] (Effective from 23 August 2021)

Assumptions and Update Logs
Electricity Risk Curve and Simulated Storage Trajectories Assumptions [ xlsx 43.29 KB ] (Updated 23 August 2021)
Electricity Risk Curve Update Log [ pdf 5.82 MB ] (Updated 23 August 2021)
Nova Energy - Thermal fuel disclosure - 1 April 2021 [ pdf 92.28 KB ]

Electricity Risk Curve and Simulated Storage Trajectories Assumptions [ pdf 357.49 KB ] (base document effective from 12 May 2021)

  Simulated Storage Trajectories Reduced Gas Demand Flexibility [ pdf 301.63 KB ] (May 2021)
  ERC Reduced Gas Demand Flexibility Scenario [ pdf 814.83 KB ] (May 2021)
  Simulated Storage Trajectories Reduced Gas Demand Flexibility [ pdf 1.47 MB ] (March 2021)
  ERC Reduced Gas Demand Flexibility Scenario [ pdf 1.59 MB ] (March 2021)
  Reduced Demand Flexibility Scenario ERC [ xlsx 13.9 KB ] data (March 2021)
  Simulated Storage Trajectories No Thermal Deratings [ pdf 323.93 KB ] (December 2020)
  ERC No Thermal Constraints Scenario [ pdf 558.69 KB ] (December 2020)

Hydro Information

For security of supply purposes, hydro storage is divided into two categories; controlled and contingent storage. Generators can use controlled storage at any time, but contingent storage may only be used during defined periods of shortage or risk of shortage. During sustained dry periods, controlled and contingent storage are important indicators of overall supply risks. Storage is expressed in gigawatt-hours – GWh (a measure of the energy that can be produced using the water).

Storage increased in the North Island and increased in the South Island over the last week, with South Island storage at 80% of full and North Island storage at 50% of full.

Lake Levels

Island Inflows and Storage

The charts below show storage over the last 13 weeks and rolling 7 day inflows for the last year in North and South Islands. 

  • Over the last week (Sunday to Sunday) available storage in the North Island has increased and the South Island has increased.
  • Inflows over the last 4 weeks (Sunday to Sunday) in the North Island have been below average and in the South Island they have been above average.
North Island South Island
Contingent Storage

Contingent storage is stored hydro that is only made available for generation at specific times to mitigate the risk of shortage. Current available contingent storage is shown on the following graph.

For more information on contingent storage and the conditions of its use, refer to the documents below.

Contingent Storage additional information [ pdf 167.95 KB ]
SOS101 - Contingent Storage [ pdf 175.52 KB ]

Market Indicators

Demand, generation mix, HVDC transfer and prices can all indicate the market response to the current security of supply climate.

Renewable generation over the last seven days was 89% of total generation, with hydro generation accounting for 62% of total generation.

Weekly Generation


Electricity consumers range from large industrial sites (the most significant is the NZAS aluminium smelter at Tiwai), down to individual households. Almost two thirds of national demand is located in the North Island. New Zealand's annual electricity consumption ('demand') is nearly 40,000 gigawatt-hours (GWh). If demand differs from expected, it may impact on security of supply.


Hydroelectric generation contributes around 60% of New Zealand's total electricity supply, with many generators of widely varying sizes distributed throughout the country.

HVDC Transfer

The ability to transfer electricity from one island to the other is an important aspect of managing security of supply, particularly as there are no thermal stations in the South Island to call upon in times of low hydro storage. Net weekly HVDC transfer is shown in the chart below with north transfer from Benmore to Haywards and south transfer in the opposite direction.

Wholesale Spot Prices

Spot prices can be an indicator of security of supply risk. Typically they rise when supply is tight, such as during 'dry years'. Weekly 7 day rolling spot prices for each island are shown in the graphs below. The corresponding prices for the previous year are also included for comparison.

Industry Workshops

We run security of supply workshops on topics of interest to the industry. The 2021 Workshops on Security of Supply are linked below.

3 Jun 21 18 May 21 3 May 21 20 Apr 21 7 April 2021 31 Mar 21 26 Feb 21

Policies, Plans and Publications

Includes information on the Annual Assessment and Transpower policies related to Security of Supply: the SoSFIP, EMP, SOROP and Outage watch List


There are currently no consultations on security of supply.