Niwa said that if the cyclone did reach New Zealand, it would produce heavy rain, wind and rough seas.
Most parts of the North Island will reach the mid-20s today, and will remain warm and settled until Thursday. There is a very small chance of a showers, Haddon said.
Auckland is likely to reach 25C today, and will get warmer as the week progresses, peaking at around 27C on Thursday.
"It won't be as hot as what we've seen over the last month or so," said Haddon.
The rest of the North Island will have similar temperatures, though it could get warmer in Gisborne and Napier, and Wellington will be slightly cooler.
FORECAST
• Whangarei
Today: 25C, cloudy, possible shower before lunch. Southeasterlies.
Tomorrow: 27C, cloudy, possible after shower. Northeast breezes.
• Auckland
Today: 25C, cloudy at time, possible isolated showers north of Waitemata Harbour. Easterlies.
Tomorrow: 25C, cloudy periods, southwest breezes.
• Tauranga
Today: 24C, fine then some cloud in the evening. Easterlies turning northerlies.
Tomorrow: 26C, morning cloud then fine. Northwest breezes.
• Taupo
Today: 24C, fine spells, light winds.
Tomorrow: 23C, morning cloud, then fine.
• Wellington
Today: 21C, fine. Northerlies developing this evening.
Tomorrow: 22C, fine. Northeasterlies.
•Christchurch
Today: 23, fine, some morning and evening cloud. Northeasterly breezes.
Tomorrow: 26C, mainly fine, some evening cloud.