This year's drought was one of New Zealand's most extreme and the worst in nearly 70 years, a study has revealed.
The comparative study, commissioned by the Ministry for Primary Industries (MPI) and undertaken by Niwa, found the drought was the worst since that of 1945-46.
It was also one of the most widespread, with only the 1972-73 drought in Wairarapa, Tasman, Otago and Southland coming close in terms of geographical spread.
The study found the cause of the drought was not the prevalent El Nino system, but slow-moving or 'blocking' high pressure systems over the Tasman Sea and New Zealand over summer.
Niwa researchers looked at their own virtual climate station network data, dating back to 1972, as well as longer-term station records that go back to the early 1940s.
They found that when measured on a drought index known as potential evapotranspiration deficit (PED) - which measures estimates of soil water content - this year's drought was the worst since 1972, and particularly serious for the North Island.
The longer-term station record calculations indicated that in some regions, it was the most severe drought since 1945-46.
The study found the most affected areas in this year's drought were southern Northland, Auckland, Waikato, Bay of Plenty, the central North Island, Gisborne, Hawkes Bay, Wairarapa and parts of the north and west of the South Island.
Niwa principal scientist Brett Mullan said the PED data from last July to May this year was the worst since 1972 for more than one third of the North Island.
He said previous severe droughts had occurred in 1972-73 and 1997-98, which were both El Nino years.
"This latest drought was different, being related to persistent high pressure centres over New Zealand during summer - a trend that is increasing according to century-long pressure records.''
MPI North Island resource policy manager Stuart Anderson said the report provided a solid picture of the drought's extent and severity. "Anecdotally we were told the drought was the worst some farmers had experienced but it is important to have this analysis to understand how dry the regions were compared to previous droughts.''
Mr Anderson said the drought had eased with the onset of winter and many farms had started to recover following good autumn conditions.
However, the economic and social impact of the drought continued to be felt around the country.
"The recent snow makes it harder for farmers to see themselves through the winter and manage feed supply and pastures.''
Mr Anderson said the adverse event declaration would remain in place until September 30, which would allow rural communities to get through winter and into early spring.
An adverse event declaration allows the Government to provide extra funding through rural support trusts, as well as tax relief and hardship payments for eligible farmers.