Dams spilling excess water

Southern hydro lakes have turned the tables on power companies as instead of too little water, there is now too much, forcing "spilling" to occur.

Several factors, including heavy rainfall, a restricted HVDC cable and the Tiwai Point aluminium smelter being at reduced capacity, meant lakes were "brimming" with water, Meridian Energy external relations manager Claire Shaw said.

A transformer at Rio Tinto's Southern smelter failed in November, putting one potline out of action and reducing the operation's power consumption by 180MW.

Warmer weather and the holiday period had also reduced residential and commercial demand.

Lake Pukaki and Lake Manapouri were both at maximum level and started to spill yesterday, while Lake Tekapo was at 108% capacity and had been spilling water for the past 15 days, Ms Shaw said.

It was part of the company's resource consent to spill when lakes reached maximum levels.

Rather than the usual concern of low lake levels, it was good news they were full, she said.

Contact Energy communications manager Jonathan Hill said water was being held in Lake Hawea and was being spilled from the Clyde and Roxburgh dams.

The situation was "not ideal" nor in the "national interest", he said.

However, rather than having an excess of power, Contact was forced to limit generation because the transmission grid was too constrained to cope with dams operating at capacity, Mr Hill said.

The situation highlighted how critical a robust transmission network was.

If it was performing properly, it would be capable of getting power to where it was needed.

Instead, "we are spilling water in the South and burning gas in the North".