The North Island is in store for heavy rain and humidity as an area of low-pressure forms this week.
Long-range data also shows a moderate risk a tropical cyclone could develop as the low pressure tracks southwards, according to WeatherWatch.
Even if a cyclone doesn't form, the West Coast and North Island can expect heavy rain over the coming week.
"This is the opposite to winter when the sub-Antarctic airflows come over NZ bringing snow to sea level, in this case instead of Antarctic air bringing Southern Ocean conditions we have tropical air bringing weather conditions Fiji and Tonga usually have," says head forecaster Philip Duncan.
"Basically this low-pressure zone is dragging down the tropics with it, like popping a water balloon. It's likely to drop a fair bit of moisture in a short period of time. The only thing left to work out is precise tracking as that determines if the rain will directly affect NZ, or mainly fall nearby out at sea."
Duncan says it's too early to tell exactly how much rain each area will get.
"We're not just watching one storm and the rain with it - we're watching to see how this large pool of tropical low pressure will interact with other systems in the NZ area. It is quite complicated so we need a few more days to be more confident of how this will pan out".
And the North Island can expect a wet end to Waitangi weekend with more rain predicted over the coming days.
