Key Points:

Rivalries are re-emerging as South Island power generators cry foul over having to foot the full costs of the inter-island power cable.

Southern generators argue the traditional reason for them paying - that they get greatest benefit by selling excess electricity to North Islanders - is outdated.

North Island generators sending power south should now have to pay their share, the companies say. The issue has been raised ahead of a conference next week in which the industry will discuss approval for Transpower to carry out a multimillion-dollar upgrade to the High Voltage Direct Current (HVDC) link through Cook Strait.

South Island generator Meridian Energy said the link should be paid for by "anyone who uses and everyone who benefits from it".

"It's an old myth now that it's used only to take South Island power north. Most of the last couple of years, it's worked the other way around," said Meridian spokesman Alan Seay.

South Island generator Trustpower has warned it may have to shelve large-scale hydro and wind generation projects because of the heavy costs of the cable.

The old days of "bucketloads" of power being sent north to make money were gone, said spokesman Graeme Purchas.

"The two islands are very complementary. Something that benefits all New Zealanders should be paid for by all New Zealanders. It should be divvied up amongst all the generators."

Electricity Commission chairman David Caygill said the commission understood the concerns of the southern generators. The unusual situation this year where power flowed from north to south almost half the time was something the commission could consider.

"We are not saying 'no, that's that. Go away. We have resolved this for all time'. Next year may well provide the breathing space when we might have the opportunity to revisit this."

Energy consultant Bryan Leyland said the cable had helped keep the South Island afloat over the past six months when hydro lakes were near record lows. While the South Island was obligated to pay a large share, he felt the North Island generators should have to share the costs.