New Zealand is in for a bit of storm next week as remnants of two cyclones are set to merge in a rare weather occurrence.

It means more widespread rain and wind - more people will receive unsettled weather as opposed to winds and rain being twice as bad, WeatherWatch analyst Philip Duncan said.

The main effect reaching New Zealand was likely to be rain, though gales were possible in some main centres.

A "worst-case" scenario would be 150km/h winds across exposed parts of the eastern Waikato, Hauraki Gulf and some parts of Northland and the Far North, Mr Duncan said.

Tropical cyclone Vania has been downgraded to a tropical depression, but is moving south.

MetService weather spokesman Bob McDavitt said that if a tropical cyclone encounters an injection of cold air as it approaches New Zealand, then it may "feed" on the difference in density so that the cold air turns the tropical moisture to rain, and that rain-making process lowers the pressure, so that the system becomes "extra windy".

Forecasters were also closely watching the newly-developed tropical cyclone Zelia, classified as a Category 1 cyclone with winds averaging 65km/h and winds gusting up to 120km/h.

Cyclone Zelia is expected to quickly move across the warm waters of the Coral Sea this weekend which will transform it into a severe category 3 cyclone by Monday morning.

Zelia will likely track south of New Caledonia on Monday and quickly catch up to the remnants of Vania by Tuesday, said Mr Duncan.

Computer models show the two weather systems merging on Tuesday and Wednesday and moving much further west of New Zealand than initially forecast, which could have a serious impact for the Nelson region.

"We may see this tropical system feeding very heavy rain into the Nelson region during the week and following the flooding in December we advise locals to be prepared for possible flooding," said Mr Duncan.

State-owned forecaster MetService said the remnants of Cyclone Vania were expected to pass near Norfolk Island tomorrow.