New Zealand declared a drought across its entire North Island for the first time in at least 30 years on Friday, with low river levels in the capital Wellington also worrying officials.

Primary Industries Minister Nathan Guy said: "It has become clear that nearly all farmers in every part of the North Island are facing very difficult dry conditions".

"Parts of the South Island are also very dry, in particular the Grey and Buller districts. We are keeping a close watch on all further regions," he said.

Guy added that while some rain was forecast this weekend, "we will need more than this to help prepare for the winter and set up for next spring".

On its website, the government said the drought had progressed rapidly and rainfall in March and April was critical as a lack of autumn rain would cause serious impacts on the next production season for farmers.

In an economic note out earlier this month, the ANZ banking group said its analysis suggested the current spate of dry weather was likely to significantly weigh on primary production and could wipe 0.5 percent off GDP by the end of the year.

Wellington City Council has called on residents to conserve water.

"Water levels in our local rivers -- the source of our water supply -- are extremely low and dropping," the council said in a statement.

"A significant reduction in demand for water will extend the number of days that back-up storage will last, so it's important to save water now."

From Saturday, there will be a ban on all outdoor water use including hoses, sprinklers and irrigation systems in Wellington to ensure there is enough water for households, businesses and public services if the dry continues.

Extra restrictions may be needed if our water reserves continue to drop.

The council, which can impose fines for breaching the ban, said it will cease the daily irrigation of sports fields and gardens and is turning off fountains and water features across the city as much as possible.