American and New Zealand data were plotting the remains of the cyclone would move into the upper North Island with severe gales and heavy rain on Friday and Saturday, Mr Duncan said.
In the past 24 hours, American and European weather modelling agencies forecast central air pressure in the low could have similar strength to a category 1 or 2 cyclone, creating sustained winds above hurricane force (120km/h), he said.
Long-range data suggested rain, desperately needed across parts of the North Island and in some parts of Canterbury, was looking possible and potentially heavy, Mr Duncan said.
Tropical lows became unpredictable when they hit New Zealand because of the transition from a warm-centred tropical storm to a cold-centred ex-cyclone, he said.
The Fiji Met Service said it had moderate to high confidence the current tropical depression near New Caledonia would become a tropical cyclone, WeatherWatch said.
MetService severe weather forecaster Oliver Druce said the earliest the weather system could affect New Zealand was Friday or Saturday. "This tropical cyclone hasn't properly formed yet and it certainly hasn't been named.
"It's expected to develop in the next day or so but it's a long way out."
Computer modelling predicted the cyclone could come down towards New Zealand, but that had yet to be confirmed, Mr Druce said.
"It's very early days. It's going to be a waiting game the next few days. The risk is there, but it's a long way off. It's very ill-defined at this stage."
Generally, across the country, the week ahead was looking "very benign", Mr Druce said. "Lots of fine weather."
