Rain would bucket down across most of the North Island on Saturday and Sunday.
Niwa described the days of wet weather as a slow-moving Tasman storm.
MetService said the north could expect significant totals on Sunday and Monday with Northland, Auckland, Coromandel and Mt Taranaki in the firing line.
Much of the South Island would also be in for a drenching with parts of Nelson seeing some of the heaviest falls from Saturday to Monday.
Only Southland, Otago and Fiordland will remain under clear skies.
The unsettled weather would continue into next week with rain falling across much of New Zealand.