Megan Woods has sent a stern letter to power company bosses, after 20-thousand households in the North Island shivered last night

Electricity was cut to some homes and hot water cylinders were turned off to deal with record demand.

The Energy Minister says she's taking the matter seriously and is asking power companies for their full cooperation.

Woods says New Zealand consumers don't expect our electricity market to fail under demand pressures at anytime, not least on a cold winter's evening.

She's asking power companies what actions they took to generate more electricity yesterday, and demanding they participate fully with any information requests.