Niwa's chief climate scientist says one-in-20-year droughts are likely to double in frequency in many parts of the country by the 2080s.
David Wratt says droughts are likely to become more frequent as the century progresses and the high-pressure belts in the subtropics expand further south and encompass New Zealand.
Niwa scientists say they believe the North Island's drought is the worst since the early 1970s.
On Friday the Buller and Grey districts of the West Coast joined the North Island when they were also declared to be in a state of drought.
Primary Industries Minister Nathan Guy says farmers requested the declaration, as conditions there were deteriorating quickly.