MetService advised city commuters to check its rain radar before leaving work today.
MetService meteorologist Mmathapelo Makgabutlane earlier said the frontal system was due to make landfall on the South Island's West Coast tomorrow, bringing with it heavy rain and gale-force winds.
But even before then showers and rain were forecast for most western parts of the country Monday thanks to a warm and humid northerly airmass from the Tasman.
Auckland would see cloudy periods and a few showers to start with but becoming possibly heavy and thundery in the afternoon and reaching a high of 23C.
There was a low risk of thunderstorms for much of the central and upper North Island and the far north of the South Island on Monday morning.
This increased to moderate by late morning from Northland, Auckland, Coromandel Peninsula and Bay of Plenty down to northern Taihape and Taranaki, including the ranges of Gisborne and Hawke's Bay.
These thunderstorms could bring downpours of up to 25mm/h.
With another system set to arrive at the end of the week, the run into Christmas is starting to look a little unsettled, although long-range forecasts at this stage are not predicting anything too major.
Tomorrow the severe weather is set to intensify, starting around Buller and Nelson before making its way up the entire country through the day.
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The heaviest rain was forecast in western and central areas. Gale-force northerlies were also predicted in places.
Makgabutlane said at this stage there were no severe weather warnings in place but this would likely change through Monday.
On Wednesday the front would be followed by widespread showers and cooler southwesterly gales.
The system would mostly have moved away from New Zealand by the end of Wednesday, but another front was likely to arrive by Thursday and Friday.
