Rain is already falling across the country but the worst is reserved for coming days.
Forecaster Ben Noll said there were worrisome trends in models projecting huge rainfall and possible flooding across the lower North Island.
The tail end of the destructive cyclone, which has claimed lives across Queensland and New South Wales, was expected to dump double Wellington's entire average April rainfall in 48 hours.
MetService said the worst weather was reserved for Tuesday and Wednesday with heavy falls across central New Zealand. There would also be strong southeasterly winds that could turn into destructive gales.
WeatherWatch.co.nz said central regions around or near Cook Strait, and those facing towards the Tasman Sea such as Nelson and Taranaki, were at risk of flooding.
Heavy rain would start falling as early as this evening in Taranaki and last until Thursday.
The region could see up to 250 mm in the next few days exceeding the average rainfall for the month.
The bad weather was likely to move on to the South Island on Wednesday and clear on Thursday.
MetService said there was still some uncertainty regarding which areas would get the worst rain and strongest winds.
At this stage Taranaki through to Nelson would have exceptionally heavy rain, while
Manawatu to Nelson would be pounded by strong gales.
There had been no warnings issued at this stage but the forecaster said the impact was likely to be large and people were urged to keep up to date with the latest forecasts and warnings and take precautions.
