"[Gita] could still be a significant and potentially dangerous low."
She said there could be gusts of more than 120km/h for the regions hit.
"It really depends how this system develops over the coming days."
The cyclone will enter the Tasman Sea over the weekend and could arrive in New Zealand on Tuesday or Wednesday next week - or even Monday, MetService meteorologist Brian Mercer said earlier.
The latest charts from Fiji Metservice show Tropical Cyclone Gita, which ravaged parts of Samoa, Tonga and southern Fiji this week, is now passing below New Caledonia.
Read more: Fijian Red Cross shares evacuation plan that saved lives on tiny islands in Cyclone Gita
The North Island will most likely be hit hardest, but if it stays west a bit longer it could also hit the south.
"We will be more certain about that in the coming days," Mercer said.
The Category 3 tropical cyclone may be downgraded to a Category 2 well before it arrives in New Zealand, but it will still hold much power.
Read more: Gita leaves some Fiji islands in the dark as Tonga picks up the pieces following mass destruction
The Fiji Metservice is now tracking the cyclone, but once it comes past 25 degrees south, it becomes New Zealand's responsibility.
"We expect that to happen tomorrow or Sunday," Mercer said.
Those still struggling to cope with the extremely humid weather at the moment should not expect relief anytime soon.
"In short, the humidity is not going anywhere," Mercer said.
"Tropical, hot and moist air remains over New Zealand, and there is not going to be any significant change until the ex-tropical cyclone passes.
"Following that, a southerly change will bring cooler air over the country.
"It will be a welcome change."
Today will be mostly fine for the North Island, with just a little cloud and fog over Hamilton and Waikato, but that should burn away during the morning.
There will be a few showers and cloudy periods on the Kapiti Coast also.
Tomorrow is expected to be sunny for most of the island, especially on the East Coast.
A front has begun moving slowly up the South Island, starting in Fiordland and Westland, where a severe weather warning is in place.
"About 120-150mm of rain is expected there by 3pm tomorrow," Mercer said.
"It is raining a bit there now but it is expected to really set in this afternoon, with potential thunderstorms."
Small patches of rain could spread over to Invercargill and Dunedin, but most of it will fall in Fiordland and on the Main Divide.
The rest of the South Island will have a slightly cloudy day, with gusty northwesterlies expected in Canterbury.
"But it should all come right by tomorrow morning with a fine day for most of the South Island."
Today's weather
Whangarei
Partly cloudy, chance morning shower. Northeasterlies. 27C high, overnight 19C.
Auckland
A fine day with some evening cloud. Light winds. 28C high, 20C overnight.
Hamilton
Fine apart from early fog. Light winds. 29C high, 16C overnight.
Tauranga
A fine day, but becoming cloudy tonight. Light winds. 27C high, 19C overnight.
New Plymouth
Fine apart from some morning cloud. Northwest breezes. 24C high, 18C overnight.
Napier
A fine day. Light winds, tending northerly from afternoon. 26C high, 18C overnight.
Wellington
Cloudy at times, and chance light showers in the north. Strong northerlies. 24C high, 19C
overnight.
Nelson
Fine spells, but chance evening showers. Northerlies. 25C high, 19C overnight.
Christchurch
Dry with increasing high cloud. Northerlies. 29C high, 18C overnight.
Dunedin
High cloud thickening. Northeasterlies. 25C high, 15C overnight.
