An extreme weather event that experts say is likely connected to climate change is expected to bring a month's worth of rain to parts of the country in a matter of days.
Atmospheric rivers are large and extremely high plumes of moisture that move in the atmosphere from the tropics to the mid-latitudes, where New Zealand is located.
Niwa meteorologist Tristan Meyers said when they hit another weather event or encounter New Zealand's mountainous terrain, vast amounts of water vapour is squeezed out, falling as heavy rain or snow.
MetService has issued a number of weather warnings ahead of the downpours, including a "red warning" for Buller and Westland. These warnings are reserved for the most extreme weather events where significant disruption is expected.
University of Otago senior lecturer in geography Dr Daniel Kingston said this event could be more damaging because it comes on top of an already exceptionally wet winter.
July was the wettest on record while more than 40 locations have experienced record or near-record rainfall, he said.
"The ground is already very wet with limited capacity to absorb further rain."
MetService has warned of a further 300-500mm of rain for Westland, especially between Bruce Bay and Otira, and 350-550mm for Buller. Red heavy rain warnings are in force for these regions until Thursday afternoon.
Meanwhile, heavy rain is also expected in Northland, Waikato, Waitomo, Taumarunui, Taupō, Taihape, Mount Taranaki and Marlborough.
Kingston said although this specific weather event had not been analysed regarding the influence of climate change, it was "more than likely playing a role".
Average air temperature had warmed by slightly more than 1C over the past century and as the atmosphere warmed it could hold more moisture, increasing the likelihood for extreme heavy rainfall events such as this.
"Sea surface temperatures around New Zealand are also warmer than average right now, which can further amplify these sort of events."
Niwa's Tristan Meyers said two bursts of rain were expected, from Tuesday-Thursday and then again from Saturday-Sunday, affecting primarily the South Island.
"All up, over a month of rainfall could fall for large parts of the West Coast, Canterbury High Country, Tasman, Marlborough and Nelson.
"This will lead to slips, and possibly flooding for some South Island rivers."
It would also melt snow which could exacerbate riverine flooding in some areas, he said.
Meyers said heavy rain was also expected for parts of the North Island, particularly Northland, Taranaki and Bay of Plenty, with the most intense downfall predicted on Wednesday.
"The science is investigating whether or not atmospheric rivers are increasing for us, but I'd speculate that it's in-line with what we would expect from climate change," he said.
"For every degree of warming, the amount of water vapour in the atmosphere can increase by about 7 per cent."