A tropical low is expected to drop large amounts of rain across much of the North Island and the top of the South Island from Monday.
The National Institute of Water and Atmospheric Research (NIWA) forecast the rain to hit Waikato and travel down the North Island's west coast and across Cook Strait to Tasman and Blenheim, spilling over Hawke's Bay and Bay of Plenty.
Principal forecasting scientist Chris Brandolino said the rain would not be as extreme as the recent falls following cyclone Debbie across the Tasman, but 200mm to 300mm was forecast and could cause flooding in some places.
"We're talking a month's worth of rain in some places. And when we get that much rain, it's got to go somewhere, all of it can't go into the ground."
MetService said there was a moderate risk of rainfall reaching warning levels from Taranaki to Hawke's Bay on Tuesday and Wednesday, as well as in Marlborough, Nelson, Buller and northern Westland on Wednesday and Thursday.
NIWA's seasonal outlook for autumn showed warmer-than-usual temperatures from Northland to Waikato, and average, or warmer temperatures everywhere else.