Cyclone Uesi is on track to "directly hit" the South Island, with experts warning the severe storm could sweep the island with flooding and thunderstorms.
The severe category 3 cyclone is moving into the north Tasman Sea on Wednesday and is heading southwards.
"The forecast isn't ideal for New Zealand's South Island with the cyclone likely to retain a lot of moisture and energy when it reaches New Zealand and hits areas recently affected by major rain and flooding," WeatherWatch warns.
"Normally this wouldn't make as much news but following recent severe rain and flooding this tropical storm needs to be monitored closely."
MetService says the centre will approach Fiordland from the northwest on Sunday, then likely stall offshore during Monday.
"Rain bands associated with the storm should sweep over the South Island from later on Sunday, possibly bringing heavy falls and severe gales for Fiordland and parts of Westland," it says on Facebook.
"Even though the storm will no longer be characterised as a tropical cyclone by then, the severity of the weather associated with it will still be significant for parts of the country still recovering from recent storms."
NIWA says there's also an increased chance of showers and thunderstorms in other parts of the country through next week, including some of the parched areas.
WeatherWatch warns says trampers planning to walk in Fiordland and surrounding National Parks should be paying "very close attention" to the forecast from this Sunday to next Wednesday and alter plans if necessary.