NIWA and Auckland University marine physics principal scientist Craig Stevens explains why Cook Strait currents are so extreme.

Cook Strait's ferocious currents have been lifting a "backbone" communications cable from the sea floor as the search begins for a heftier replacement.

Transpower – the state-owned enterprise that owns and runs the national grid – is now in the process of finding a company to supply up to 90 kilometres of fibre-optic cable to link the North and South islands.

Tender documents show the existing cable is too light and lifts from the seabed because of the strait's high tidal flow. Burying the cables was not an option, as they were too close to the interisland electricity cables.

SUPPLIED The relaying of a new part of a Cook Strait power cable in March 2005.

Transpower is now investigating the cost for either one or two cables that were at least three times heavier, "specifically designed for the conditions in Cook Strait".

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The new cables, despite weighing up to 20 kilograms a metre, would also need to be flexible enough to contour to the sea floor.

STUFF A 1991 newspaper clipping from when the power and fibre optic cables were laid between the North and South Islands.

In a statement, Transpower's Cobus Nel said the two fibre-optic cables formed "part of our communications backbone".

"From our annual inspections, we have identified that one of the Transpower fibre-optic cables' condition is deteriorating, indicating that it is nearing end of life and a replacement strategy has been established (as per normal business practice with all our assets).

"We have discovered certain sections of these cables are moving more than other sections, due to tidal movements in the strait: this has caused some corrosion in parts of the cable."

STUFF Fibre optic cable that was laid for a separate cable across Cook Strait in 2001.

The cables were originally installed in 1992, and replaced in 2003. One of the cables – or perhaps two if it was more cost-effective – would likely be replaced in the next one to two years, he said.

"Given our knowledge of the condition of these cables, and our contingency measures, we do not believe there is any imminent risk to the operation of the national grid."

Niwa and Auckland University marine physics principal scientist Craig Stevens said the Cook Strait tides ripped through at up to three metres a second, or about 10kmh.

SUPPLIED A damaged high-voltage cable being retrieved from Cook Strait at Oteranga Bay near Wellington. The fibre-optic cables are laid nearby.

"So if you consider that each cubic metre of water weights about a tonne, you start to take that sort of force pretty seriously."

The extreme current was caused by New Zealand's size and the rotation of the Earth, causing an effective tidal wave working its way anticlockwise around the country.

When it reached Cook Strait, which could have almost high tide on one side and low on the other, it had to squeeze through a narrow gap.

Telecommunications Users Association of New Zealand chief executive Craig Young said there were multiple cables between the North and South islands so an outage on one would not cause a major issue.

But having cables lying on the sea floor made them vulnerable – for example, if a boat's anchor pulled them up.

Fibre-optic cables were essentially a couple of thin pieces of wire surrounded by a thick protective layer, he said.

A Spark spokeswoman said it owned one of the three cables between the North and South islands, which was upgraded last July. It sometimes used the Transpower cable when it needed extra space.

Asked if a failure to the Transpower cable could cut off the South Island's communications, she said that would not be the case.