Wholesale electricity prices spiked sharply upwards on Friday, more than doubling within an hour.
They later slipped back, but still remained above average in the North Island and very high in the South island.
Auckland energy analyst Brian Leyland blames the jump on South Island hydro operators raising the price to choke off demand to protect low water inflows into hydro storage lakes.
Mr Leyland says that was exacerbated by a planned shutdown of the Cook Strait cable for routine maintenance.
"Yesterday prices were between 30 and 40 cents a kilowatt hour. Normal prices for this time of the year would be between 5 and 8 cents.
"The direct current link between the two islands was out because of work on upgrading it. The South Island lakes are very low and generators are getting nervous about the amount of storage they have got."
Hydrological data show inflows into hydro storage lakes substantially below average for this time of year.
The increases are separate from price rises announced this week and blamed on higher transmission costs by national grid operator Transpower.