The first of Cyclone Hola's winds will be felt tonight, before its full force hits on Monday.
The cyclone's winds and rains have left at least one person dead and two others seriously injured in Vanuatu, and it is now tracking towards the upper North Island of New Zealand.
Metservice meteorologist April Clark said there would be heavy rain and severe gales above 90km/h, with even stronger gusts of up to 180km/h.
There was still uncertainty about where the cyclone would hit.
"The low centre is still a little bit varied in terms of how far over the upper North Island it might be, or maybe just skirting the north-east coast of the North Island," she said.
"Since it's such a compact system, the most severe weather is quite confined. So where that low centre ends up will make a huge difference in terms of impacts on New Zealand."
Wind and rain will start picking up tonight through Northland, before the cyclone hits on Monday and Tuesday.
Metservice will continue to issue updates as the cyclone's path becomes clearer.
It is warning that significant damage and disruption is possible, including downed power lines, flooding, slips, and damage across roads, and large waves affecting low-lying coastal areas.
Those in the upper North Island were being urged to use yesterday's relative calm to prepare for the storm damage.
Northland, Auckland, Coromandel Peninsula, Waikato, Bay of Plenty, and Gisborne are all expected to get at least some of the severe weather on Monday.
It is likely the heavy rain and gales will hit Waitomo and Taranaki across to Hawke's Bay later on Monday and into early Tuesday.
It is unlikely to go as far south as Wairarapa.
The tropical cyclone has battered Vanuatu with winds of up to 230km/h.
The roof of a classroom was blown off and some houses collapsed in Vanuatu. There was also one death from a falling tree reported.
The possibility of more severe weather is not good for those in the central North Island, who are still in clean-up mode after a sodden couple of days.
On Thursday heavy rain and flooding damaged more than 80 homes in Hawke's Bay and washed out sections of rail line.
It is expected to take several weeks to clear some roads of damage.
Niwa announced last year there was an increased risk of tropical cyclones for New Zealand this season, because of La Nina weather conditions.
A "mini-whirlwind" lifted the roof from a West Auckland factory yesterday and dumped it on power lines and cut electricity to the area, Fire and Emergency said.
Fire and Emergency shift leader Craig Dally said they were alerted to reports of an explosion in Henderson Valley Rd just after 4.30pm.
But the incident was actually said to have been caused by a weather event.
"A mini-whirlwind lifted the roof of a factory and that came into contact with power lines ... it sounds like it did spark because we were called to an explosion."
No one was hurt in the incident, but power was cut to parts of west Auckland.