Gita is expected to re-curve towards the southeast today and track towards New Zealand while transforming into an ex-tropical cyclone.
"It has turned southeast now, and the weather models are quite on agreement it will come through coastal Buller and Nelson, and head east and southeast towards Cook Strait and continue pretty rapidly from there, from Tuesday and early Wednesday.
"Unfortunately some of the areas hit by Fehi will see significant rainfall this time as well."
Mayors and council chairs in the West Coast region met on Sunday to discuss storm preparations, and would meet later again today to discuss the latest predictions.
Grey District Mayor Tony Kokshoorn said officials were considering declaring a state of emergency even before the cyclone arrives.
Kokshoorn told Newstalk ZB that the region was still recovering from the cyclone from a few weeks ago - with some roofs still off buildings in downtown Greymouth.
He says while the situation's serious and they don't want to be alarmist, they may call a state of emergency this evening after meeting at 5pm.
Declaring a state of emergency empowers officials to close or restrict roads or public places, remove dangerous structures, provide first aid, food and shelter and enter premises to rescue people.
Read more: Everything you need to know about Cyclone Gita including how it might affect NZ
As the ex-cyclone approaches New Zealand it will combine with several systems already above the country, bringing heavy rain from South Taranaki through Wellington, Nelson and down to Buller through to tomorrow morning.
Rainfall of more than 100mm will be common, bringing significant flooding in the ranges, Zachar said.
Storm-force winds will hold off until Tuesday afternoon.
"Some models are indicating northwesterlies will be hitting from Taranaki to north of Christchurch, with wind gusts exceeding 120km/h common in exposed places, and much higher gusts possible as well."
North to northwest swells of 5 to 6 metres are expected to arrive on Tuesday evening through to Wednesday morning.
"But then we have the wind waves, so a combined height of 8-9m near the centre. It is certainly a potent storm."
Much of the swell will be funnelled through the Stephens Island/Cook Strait area, with 5-6m swells expected to batter the South Island's northern West Coast.
"We are going to see some significant waves coming through. Adding the wind on top, it will be quite choppy too. You certainly won't want to be on the ocean."
There is also a possibility of unseasonable snow in the Canterbury high country, as the cyclone drags cool air with it.
It may affect some of the alpine roads around Canterbury.
Metservice would be updating its severe weather warning for the storm later today.
Civil Defence is warning Kiwis to be prepared for the possibility of power cuts, water outages and road closures.
The Ministry of Civil Defence and Emergency Management (MCDEM) said people should get together with family or housemates to make emergency plans.
MCDEM advised people to secure, or move inside, anything that could cause damage in strong winds, as well as closing windows and curtains to prevent injury from breaking windows.
People should stay inside and bring pets inside and if they have to leave, take them with.
Listen to the radio and follow the instructions of emergency services and avoiding non-essential travel during severe weather was essential, the Ministry said.
It was a fine weekend for much of the country, with Gisborne and Hastings cracking the 30C mark.
"It is really the calm before the storm," Zachar said.
Today will begin fine for most places, with cloud starting to increase through the day, and rainfall coming in parts of the country later in the day.
Some good weather news however, is that post-Gita will see an end to the humid flow drenching the country over the past few weeks.
"After Gita a cooler air mass will come through from the south. It will be a welcome relief compared to the last few weeks," Zachar said.
Today's weather
• Whangarei
Cloudy at times, chance shower. Northerlies. 28C high, 21C overnight.
• Auckland
Cloudy at times, a few showers late morning or in the afternoon. Light winds. 27C high, 21C overnight.
• Hamilton
Often cloudy. A few showers developing early afternoon. Light winds. 28C high, 19C overnight.
• Tauranga
Often cloudy. A few showers developing early afternoon. Northerly developing afternoon. 27C high, 21C overnight.
• New Plymouth
Often cloudy. A few showers developing in the afternoon. Northerly breezes. 25C high, 20C overnight.
• Napier
High cloud gradually increasing, chance evening shower. Light northeast turning northwest evening. 30C high, 21C overnight.
• Wellington
Partly cloudy, chance shower in the north. Rain developing everywhere late evening. Gusty northerlies. 24C high, 17C overnight.
• Nelson
High cloud gradually increasing and rain developing in the evening. Light winds and sea breezes. 25C high, 17C overnight.
• Christchurch
Fine with northwesterlies, then occasional rain from late afternoon with a southerly change. 29C high, 12C overnight.
• Dunedin
Morning rain, then partly cloudy with a few showers. Southwesterlies. 19C high, 11C overnight.
