Electricity grid operator Transpower is starting a $415 million project to bolster supplies to Auckland and Northland with a high-voltage underground line stretching from Pakuranga to Albany.

The cables will close an electricity loop around Auckland, reducing reliance on the existing single corridor through the west.

Impetus for the project came from a dramatic six-hour blackout of much of Auckland in 2006, caused by the failure of a small corroded shackle on a transmission line near the Otahuhu B substation, the single point of entry at that time for electricity supply to the region.

Although much of the new 37km electricity route is through dedicated transmission corridors, Transpower warned Auckland Council members yesterday that some traffic disruption was unavoidable in streets along the way.


"This is a very major cable project we are starting and which you'll see in the streets over the next year," chief executive Patrick Strange told the council's regional development and operations committee.

The greatest impact on traffic will be along Waipuna Rd in Mt Wellington from January and Ti Rakau Drive in Pakuranga from about May, although Dr Strange said two narrow lanes would be kept open in each direction.

Work has started on trenches through which 220,000-volt cables will be laid from Albany to the Northern Busway.

The cables will then be pulled under the busway - through ducts installed when it was built - to a new substation in Wairau Rd in Glenfield.

The first part of the operation will cross several streets, including Rosedale Rd.

Strengthening will also start next month on the harbour bridge to prepare for cables to be strung underneath it on the way from another new substation, being built near the bottom of Hobson St in a joint venture with power supplier Vector. Ducts have been laid from that site to the bridge through Fanshawe St and Westhaven.

Transpower will also be able to run cables through Vector's 9km service tunnel from Hobson St to Penrose.

Next month, Transpower will start dismantling an unsightly 110,000-volt transmission line from Pakuranga to Penrose.

But overhead transmission lines in other parts of Auckland, including Onehunga and Mt Roskill, will remain for now as Transpower is committed to laying cables only for new capacity.