The Canterbury Regional Council is warning farmers and holidaymakers to watch out for rapidly rising river levels after heavy rain in the Southern Alps.
About 400 millimetres of rain has fallen in the upper catchment of the Rangitata River in the past week, making it peak at 10 times the normal flow.
More heavy rain is forecast for the Main Divide area over the next few days, and Meridian Energy has warned it may have to spill water from the hydro lakes Pukaki and Tekapo.
Wholesale electricity prices quadrupled over the last month because water was being held back in the system in case rain did not eventuate, but the situation is now reversing.
Meridian says lakes that had been below average are now full to overflowing.
The regional council says anyone camping or fishing on the Rangitata or Tekapo rivers will need to take extreme care. It says farmers and other landowners next to the river should monitor the situation closely.