Forecasters are warning that Cyclone Vaianu is increasingly likely to hit the country this weekend and the National Emergency Management Agency (NEMA) says people should act now.
MetService says the incoming Tropical Cyclone Vaianu could bring life-threatening winds to the North Island on Sunday.
MetService has issued a preliminary severe weather watch for the entire North Island.
Prime Minister Christopher Luxon said the agencies were on "full notice" over the weather system.
"This is shaping to be a very significant and damaging weather event, and I just think obviously with big impact on the North Island, particularly around Sunday, it's a great opportunity to remind New Zealanders to do everything they can.
"Stock up, get their supplies in order, get items that may be loose in their backyards tied down and in secure storage."
He said it was important people did everything they could to prepare.
NEMA director of civil defence emergency management John Price said people should take the time to act before Cyclone Vaianu arrived.
"Act now and not later - later may be too late!"
People should clear their drains of any debris and tie down or bring inside anything that strong winds could break or pick up, as well as keeping up to date with the latest MetService weather information and any information from their local civil defence emergency management group, he said.
"Trust your danger sense, and don't wait for official warnings. Do not enter floodwaters, as they are dangerous and can kill you," he said in a statement.
The ground was still saturated after the recent heavy rain fall which meant that landslides were likely to happen in some places, he said.
"Watch for warning signs like small slips or rockfalls, water flowing out of a slope, trees or fences that start to tilt or doors and window frames that start to stick. Also take care around trees which may become unstable and fall."
If it hits, it's likely to "bring damaging, potentially life-threatening winds". As well as the gales, heavy rain and hazardous coastal conditions were likely in some areas, MetService said in its latest update.
Some areas are likely to be upgraded to orange or red warnings as the cyclone approaches, it says.
Heavy rain watches or warnings are also likely.
However, the cyclone's exact track remains uncertain, so it's still too early to forecast which parts of the North Island would be most affected.
"The areas most impacted by Cyclone Vaianu will be heavily dependent on the movement of the system," meteorologist John Law says.
"In the coming days, as clarity in the forecasts increase, the Severe Weather Watch will be updated and Severe Weather Warnings will be issued highlighting the areas most at risk."
Significant and damaging
Acting Emergency Management Minister Chris Penk said Cyclone Vaianu had the potential to be significant and damaging.
He said the storm would be unwelcome for communities already hit by earlier severe weather.
More rain on ground that was already saturated would mean a bigger risk for falling trees, landslides, flooding and dangerous rivers, he said.
The government was asking people to take sensible precautions and people in the North Island should be prepared for power cuts, Penk said.
System could cause power outages, slips, road closures
Meteorologist Alanna Burrows said the cyclone - currently south of Fiji - was category three, and had winds up to 130km/h at its centre.
Burrows said forecasters were very concerned about it because it had the potential of bringing "damaging, potentially life-threatening winds, heavy rain and hazardous coastal conditions".
The system might have a severe impact on parts of the North Island - including power outages, falling trees, slips, road closures, and isolated communities.
Burrows said it was too soon to put a figure on the wind speeds and rainfall amount.
Earth Sciences New Zealand said its modelling also indicated the cyclone would most likely reach the North Island on Sunday.
When it nears New Zealand, the heaviest rain is expected across the north and east of the North Island. The areas that will be impacted and the intensity of those impacts will be heavily dependent on the path that TC Vaianu takes as it approaches New Zealand, MetService said.
Law said "a very close eye" was being kept on the system.
"The exact path and intensity of the storm as it heads towards us in New Zealand is yet to be determined, however, it does look like we will be seeing some impacts from this system during the weekend."
Cyclone Maila now at Category 5 strength
Another cyclone is also active in the Pacific.
Severe Tropical Cyclone Maila has been upgraded to a Category 5 system today, with winds near the centre of 215km/h and wind gusts up to 295km/h.
Australia's Bureau of Meteorology said Maila, currently located in the Solomon Sea, will move towards the Far North Queensland coast over the weekend.
Meanwhile, MetService has an orange heavy rain warning in place on Wednesday in Bay of Plenty west of Whakatāne.
The warning is in place until midnight for Bay of Plenty, including Rotorua.
Tauranga City Council is urging any residents in the region to evacuate if they are worried about possible landslides.
There's also a heavy rain watch for Waikato, Waitomo, Taumarunui and Taupō until midnight, and a heavy rain watch for Bay of Plenty about and east of Whakatāne until noon on Thursday.
