Parts of the South Island are being lashed by heavy rain and possible snow, while the North is hammered by gale-force winds.
An "active front" is moving up the country on Tuesday, also bringing a thunderstorm risk to the west of both islands.
The South Island's West Coast has already borne the brunt of the weather system after being slammed by rain in the past 12 hours.
Snow is also expected in Southland and central Otago as a "cold winter blast" arrives, MetService says.
A heavy rain watch is in place for the Canterbury headwaters and the Westland ranges. Metservice says flooding, slips, and thunderstorms are all likely.
The Crown Range and Milford Rds both have snowfall warnings.
The North Island, especially Auckland, could be subject to gale force winds on Wednesday. The forecast for the city on Wednesday is heavy showers and strong southwest winds.
Coastal areas in Raglan, Waikato, and Auckland should expect waves as high as nine metres, WeatherWatch warns.
"These are exceptionally large waves and people should avoid being too near the water," said head forecaster Philip Duncan.
"This week is a classic winter setup although the temperatures won't be too brutal despite a nationwide southerly coming in for Tuesday and Wednesday."
A strong wind watch is in place for the entire top of the North Island starting in the Coromandel, according to Metservice. Heavy rain and wind watches will be in place in Taranaki until at least 2pm Tuesday.
The wind and rain will begin to clear on Thursday and Friday. It follows a weekend of wild weather with a one-in-500-year downpour in Northland causing severe flooding, slips, and road closures.
