Transpower says the power emergency it declared in the North Island was due to a fault on one of the two Cook Strait cables which brings electricity up from the South Island.

It issued a grid emergency notice at 7.15am warning there was a risk of insufficient generation to meet demand in the North Island due to the a fault on the HVDC cable.

Some power was still flowing across the cable but a second fault could have suddenly stop the flow of electricity, the operator says.

Transpower chief executive Alison Andrew told Morning Report the problem has been fixed and the system has been restored.

"We dropped about 180 megawatts of controllable load - that's things like hot water heating - managed with the distribution companies,

"We're now restoring all that power back so the system is back in full strength."

She says the fault was on a piece of equipment on the receiving end of the cable, and its cause is not yet known.

"We obviously have to do do a full investigation."

The power system has been "tight" this winter, she says, and there has been situations where it has less extra capacity than Transpower would like, she says.

Alison did not not believe the fault was due to the extra load being brought up from the South Island in the unusually cold spring weather.

Sh says the system has worked as it should and consumers would not have noticed any impact.

This morning's emergency notices came after a sudden cold snap moved up the country in recent days.

The cold snap was lingering early on Friday with below-zero temperatures in Rotorua, which was on -3C, and Hamilton (-2C) while in the south Blenheim and Queenstown were on -1C at about 6.30am.

-RNZ.