The heaviest rain is expected in Tasman, northwest of Motueka, where up to 170mm could fall around the ranges, MetService said.
An orange heavy rain warning is in place there from 10am Sunday to 1am Monday.
"Heavy rain may cause streams and rivers to rise rapidly," the forecaster said. "Surface flooding and slips are also possible and driving conditions may be hazardous."
There are further orange heavy rain warnings in place for the Richmond and Bryant Ranges and the Rai Valley from midday Sunday to 3am Monday, the Paparoa Range and the ranges of the Buller District south of Seddonville and northwest of Reefton from 10am-10pm Sunday, and the ranges of the Westland District from 8am-11pm Sunday.
Heavy rain watches have been issued for Mt Taranaki, the Tararua Range and the Canterbury headwaters on Sunday, while a strong wind watch has been issued for Wellington, including the Remutaka Hill Road.
MetService said the culprits were a "strong and moist" northerly flow, followed by a "broad trough of low pressure".
"These weather systems are forecast to bring a brief period of heavy rain to northern and western regions, with the largest accumulations likely about the ranges in the north and west of the South Island.
"Gale force northerly winds are also likely about exposed parts of central New Zealand for a time."
As well as wind and rain, there would be unseasonably warm temperatures, MetService meteorologist Mmathapelo Makgabutlane said.
Sunday night into Monday morning would see many places with double-digit minimum temperatures, significantly higher than the norm for this time of year.
"Daytime temperatures will also feel unseasonably warm. Napier and Hastings are projected to reach 22C on Monday, due largely to strong northwesterly winds rather than abundant sunshine," she said. "However, there'll be moments of sunshine for much of Aotearoa New Zealand before then."
RNZ
