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South Islanders are lucky the wind is not blowing, as four steel guy-ropes are all that keeps North Island power flowing south this weekend.

Engineers found on Thursday night that one of the Marlborough Sounds pylons carrying the high-voltage, direct-current (HVDC) inter-island cables from the North Island was buckling because wet ground beneath it had slipped.

Transpower, the State-owned national grid company, yesterday installed guy-ropes embedded in concrete to support the warped tower, between the HVDC's southern terminal at Fighting Bay and Port Underwood, before a full assessment of the damage on Monday.

Transpower spokeswoman Rebecca Wilson said engineers were confident the pylon would not collapse over the weekend and there was no risk to South Island supply.

South Islanders have been relying on power sent through the Cook Strait cable because generation from critically low southern hydro lakes is unable to meet demand.

Up to 500 megawatts (MW) of electricity is being sent south across Cook Strait to make up the shortfall, and industry experts have warned that any sudden failure of the HVDC link would black out a city about the size of Christchurch.

Ms Wilson said 414MW was transferred south yesterday afternoon, and steady supply was not jeopardised by the problem.

Transpower was inspecting other pylons and their foundations in the area for signs of damage as a result of extreme wet weather in the past month.

Pylons supporting the HVDC cables have collapsed in the past, breaking the link between north and south.

Auckland electricity consultant Bryan Leyland told The Press the DC link was not something easily duplicated and total failure was a possibility.

"They're probably right when they say they can hold it up with stays and things like that. Lucky the wind wasn't blowing, too," he said.

Meridian Energy spokesman Alan Seay said the incident showed how vulnerable the South Island was, when it was relying on North Island power.

"It can be a very tenuous link."