Rain intensity could hit 20mm to 30mm per hour about the ranges Monday and 30mm to 40mm per hour on Tuesday.
Up to 600mm was forecast about the headwaters of the Canterbury Lakes and Rivers south of Arthurs Pass for the same period, and up to 500mm about the headwaters of the Otago lakes and rivers to 5pm Tuesday.
Parts of Fiordland could see up to 400mm by 1pm tomorrow.
The Canterbury high country could also see wind gusts to 130km/h until 11pm tomorrow.
The wild weather was being driven by warm, moist northwesterlies ahead of an active front over the Tasman Sea.
The cold front would slowly sweep over the South Island before stalling over the North Island about midweek.
NIWA said the "recipe" for the prolific rainfall was a combination of an atmospheric river extending from Australian cyclones, extra energy from the Tasman Sea marine heatwave, and a strong low pressure system siphoning moisture toward New Zealand.
MetService meteorologist Stephen Glassey said once the system arrived in the North Island from Wednesday it would have severely weakened but still bring some rain and showers to places.
In Auckland, weather consisting of cloudy periods and isolated showers could be expected over Monday and Tuesday with temperatures hovering around 24C.
"As [the front] stalls, it's going to bring more rain and shower activity across the North Island," MetService meteorologist Josh Griffin said.
"In Auckland ... there is some rain developing late Wednesday into Thursday and it'll kind of be cloudy and a bit showery for the Auckland area towards the end of the week."
However, the time has not yet come to pack away the shorts and T-shirts with New Zealand's long, hot summer set to linger.
Temperatures would remain consistently in the mid to high 20s for the upper North Island and in the low 20s for much of the rest of the country.
On Monday eastern parts of the country were in for some warm and dry weather. In the South Island the east coast was set to cook, thanks to the foehn effect driving warm and dry air from the Southern Alps. Christchurch was forecast to hit 29C.
A high-pressure system would bring settled weather to the country this month and despite the arrival of the cold front midweek, the weather will remain warm.
The continuing mostly settled, warm weather might be bringing smiles to many, but it was no anomaly.
"It seems to be nothing out of the ordinary for this time of year ... it's not going to be getting cold just yet," Griffin said.
Meanwhile, WeatherWatch NZ was predicting the tail of ex-cyclone Trevor could drift east and join forces with a cold front in the Tasman Sea on Friday, sparking a new rain band and possibly a new low pressure system in the New Zealand area.
"It's important because it may bring rain relief to parched parts of the North Island in the first few days of April."
