Northlanders should prepare for possible water shortages and restrictions if the region does not receive between 80mm and 100mm of rain in the next few weeks.
The Northland Regional Council said low rainfall, high temperatures and persistent westerly winds this month had resulted in low soil moisture and reduced water intake at catchments.
Areas most at risk include southern and western areas of Kaipara from Kaihu to Pouto Point, central areas south of Kaikohe, west of Whangarei, and Bream Bay.
Regional council hydrologist Dale Hansen said the situation was likely to get worse in February and March if Northland did not receive adequate rain in the coming weeks.
But Mr Hansen said people should not panic at this stage. "This is a heads-up; it's about giving people the information they need to ensure they're prepared early and thinking ahead."
The National Institute of Water and Atmospheric Research (Niwa) climate outlook for Northland for the next three months shows temperatures and rainfall (50mm) likely to be near-average.
The regional council yesterday started sending out letters to about 400 water use resource consent holders, from individuals taking several thousand litres a day to some major users taking hundreds of thousands of litres, urging them to closely monitor their usage.
Mr Hansen has encouraged Northlanders in affected areas to conserve water wherever practical and prepare for possible water shortages and restrictions.
As well as urging consent holders to conserve water, he said they were also being asked to ensure their water use, stream flows, groundwater and lake levels were closely monitored. The regional council has also advised them to plan for shortages, including investigating alternative supplies or the possibility of water rationing, to ensure they meet consent conditions if things did not improve over the coming weeks.
He says the regional council will continue to closely monitor the situation and update affected parties, including regular contact with district councils responsible for town water supplies.
Whangarei District Council water services manager Andrew Venmore said due to significant rain around Christmas, the Whau Valley dam was 90 per cent full and Wilson dam 97 per cent full.
"By comparison, we were at 72 per cent in January 2010, so we don't envisage any problems during the course of the summer. Even so, we will continue to monitor the draw closely.
"Our advice to our domestic water supply users is that there is never a good reason to waste water."
The Far North District Council is also not proposing to introduce water restrictions at this stage.
Information on rainfall, river levels and flows can be found on the regional council's website at www.nrc.govt.nz/riversandrain