A deluge of rain is expected in the north of the North Island tonight, with 70 to 100mm forecast to fall in Auckland, Northland and the Coromandel Peninsula.
Despite being downgraded to "former" cyclone, Evan continues to move southwest towards New Zealand, directing a moist tropical airmass onto the North Island.
Metservice says rainfall intensities overnight are likely to "spike" at times, and rivers and streams may rise rapidly, making slips and and surface flooding more likely.
High winds are also expected, and will reach gale force at times, although there is only a low risk of severe gale force winds affecting the far north.
The heaviest falls of rain are likely to be in eastern Northland, north of Whangaparaoa, Great Barrier Island, and the Coromandel Peninsula north of Tairua.
Wet weather is expected to continue in the north throughout Monday and into Christmas Day, while the weather system moves south, bringing rain from Waitomo to Wellington, the central high country and in Wairarapa.
There is also a low risk of heavy rain over the South Island in Westland, Buller, Nelson, Marlborough and Canterbury.
On Thursday the low and associated trough are expected to move away to the east and a ridge should bring settled weather to the country.
Holidaymakers and trampers are urged to keep up to date with the latest forecasts as there is a potential for localised flooding and disruption to transport.
The approach of the weather system that was Tropical Cyclone Evan has seen many holidaymakers cancel their plans to travel north, and Far North mayor Wayne Brown says a number of people who had booked to stay in the region haven't turned up.
The cyclone devastated parts of Samoa and Fiji last week, causing thousands of people to be evacuated to emergency shelters.
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