New Zealanders are evacuating towns along the North Island's east coast, ahead of what is being called the worst storm in generations.
Cyclone Cook is expected to make landfall at about 18:00 local time (06:00 GMT), before barrelling down to hit the South Island on Friday.
Authorities have warned of high waves, storm surges, and landslides.
It comes after severe floods caused by the remnants of Cyclone Debbie hit some parts of the country.
Parts of New Zealand's North Island are now under a state of emergency, with residents in low-lying Coromandel, which has already seen landslips and closed roads, told to leave immediately.
The BBC's Hywel Griffith says the Bay of Plenty, which suffered flooding last week caused by the tail end of Cyclone Debbie, is expecting 300mm (11in) of rain to fall, which means some houses may be flooded again.
The storm is expected to bring gales of up to 150km/h (93 mph).
Schools and businesses across the island have been shut early while residents in Auckland, which has seen flooding, have been stocking up on emergency supplies.
The cyclone formed around Vanuatu on Sunday and moved towards New Caledonia, bringing heavy rain and winds and causing cuts to power and water supplies, reported Newshub.
New Zealand weather officials say that Cyclone Cook will be the worst to hit the country since 1968.
They have also warned that it is a ‘really tightly packed cyclone’ that will bring a ‘phenomenal’ amount of rain and wind, reported The New Zealand Herald, compared to Cyclone Debbie which was more spread out.
Cyclone Debbie hit Australia at the end of March, before its remnants drifted towards New Zealand, bringing large amounts of rain that caused severe flooding.
It soaked New Zealand cities like Wellington and Auckland - which saw its wettest March on record - and prompted the entire town of Edgecumbe to be evacuated.
Authorities are now worried about how Cyclone Cook will impact land that is already saturated from heavy rains.
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