The merger of two ex-tropical cyclones and a low above the skies of New Zealand today may actually decrease the predicted severe winds in some parts of the country, weather forecasters say.

MetService has issued a severe weather warning for Marlborough, Nelson, Buller, Westland, Taranaki, Wanganui, Manawatu, Wairarapa, Wellington, Waikato, Waitomo, Bay Of Plenty Rotorua, Taupo, Taumarunui, Gisborne, and Taihape.

Zelia is forecast to pass west of Northland and Auckland this afternoon and evening, move over the southern North Island tonight and pass west of the Chatham Islands late tomorrow.

MetService is warning severe gales can be expected in exposed parts of the North Island and the top of the South Island. Trees, powerlines and insecure roofs could be damaged and driving conditions could be hazardous, it said.

According to WeatherWatch.co.nz forecasters, the merger of these three systems may actually help decrease the predicted severe winds over some parts of the country.

"The air pressure is falling across New Zealand and the area of low pressure is increasing. That makes for unstable conditions but actually helps lower the wind speeds," head forecaster Philip Duncan said. "The energy of these three lows is being dispersed across an area that's over 2500kms long".

Yesterday Zelia's destructive winds covered an area only about 100km wide, making it a small cyclone in size, but still an intense one. Mr Duncan said the narrow window of gales makes it difficult to predict where landfall will be and which areas will be worst hit.

But now a new low is forming, absorbing the energy of all systems and spreading that energy further afield, he said.

WeatherWatch.co.nz maintained wind would never be the main feature of these systems, with Zelia continuing to rapidly fall apart as predicted.

MetService forecaster Paul Mallinson said the path of Zelia is difficult to determine, but the lows should bring heavy rain and northerly gales over much of northern and central New Zealand today and early tomorrow.

The heaviest falls of 200 to 250mm are expected near the ranges of Westland, Buller and Nelson north of about Franz Josef and 70 to 150mm falls can be expected about the Marlborough Sounds, in the southwest of the North Island, and in a belt from Taranaki to southern Waikato across the central North Island to eastern Bay of Plenty and the north of the Gisborne region.

MetService warned streams and rivers may rise quickly and there are likely to be slips and surface flooding on some roads.

The strongest winds are likely to be from Taranaki and Taumarunui to Taihape and inland Whanganui, with gusts around 130 km/h in exposed places this evening.

Several flights have been cancelled at Tauranga Airport because of misty rain.

Flights to Auckland, Wellington and Christchurch scheduled to depart before midday were cancelled. Those scheduled to leave from 1pm are on time.

The lows have brought with them humid conditions, with an uncomfortable night for many across the country. In Auckland humidity reached 100 per cent with foggy conditions reported across the region. Auckland currently has 98 per cent humidity, Mr Duncan said. Similar levels have been reported throughout the country.

The two former cyclones may have a long-term impact on New Zealand, with WeatherWatch.co.nz predicting the sub-tropics will be lingering around the upper North Island for the rest of the month.

"Zelia and Vania have help pulled the tropics down over the upper North Island so we can expect more muggy, wet weather this weekend and into the next 10 days," Mr Duncan said.

He likened it to a bath of cold water with a hot tap running at one end - "the ex-cyclones were the hands that helped scoop the hot water and spread it across the bath".