Contact Energy is warning South Islanders of rising power prices.
An analysis by the company shows two separate energy markets developing - with significant price differences for power in the North and South Islands.
Contact Energy's communications manager Jonathan Hill said electricity demand had been growing rapidly inthe South Island over the past ten years, but generation had grown by just 185-megawatts.
That compared to more than 2,000 megawatts in the North Island.
Mr Hill said much of the mainland's power was now coming from the North Island, which was a problem for the ageing Cook Strait cable.
"The impact of that is higher wholesale electricity prices in the South Island from which retailers like Contact need to buy electricity to supply to their customers.
"So we're seeing higher average wholesale prices in the South Island which will inevitably flow through to consumers," he said.
