The blast furnace door is open today and pointing at the Central Otago town of Alexandra, where New Zealand's top temperature of 35C is predicted.

Nearby Wanaka took the national top spot yesterday, with 33.5C, and is forecast to reach 34C today.

Aucklanders can expect another hot day too; 29C is the forecast high, up from 28C yesterday. At midnight last night the city was a balmy 22C.

MetService meteorologist Kyle Lee said the high pressure zone over New Zealand would continue to deliver mainly fine conditions for many areas, and warm to hot temperatures.

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Light winds, mixed with heat and humidity are expected to brew up some isolated showers or thunderstorms in the central North Island and inland parts of the South Island from this afternoon. The risk of thunderstorms in many of these areas is low, but reaches moderate in the western part of the central North Island.

Isolated showers are also possible today in eastern parts of Northland and the Coromandel Peninsula.

It may pay to make the most of the fine weather - a cyclone that could include New Zealand is forming up in the tropics.

WeatherWatch said the cyclone was expected to develop today near New Caledonia before quickly weakening and drifting south to New Zealand on Thursday.

As an ex-tropical low, it is expected to bring rain to much of New Zealand and possibly localised gales, although its likely path is uncertain.

"This tropical low will have the potential to cause localised flooding in western- and northern-facing regions of both islands," WeatherWatch said.

"It should be short-lived - most models pick it moving across New Zealand in 24 hours or so."

"There could be two more tropical storms behind this one - but latest guidance suggests they may slide just east of New Zealand."

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Today's forecast:

Auckland
29C Fine apart from areas of morning cloud. Easterly breezes.

Hamilton
30C. Mostly sunny, chance afternoon shower. Light winds.

Tauranga
27C. Fine. Light winds and sea breezes.

Wellington
28C. Areas of low cloud clearing this morning to a fine day. Light winds.

Christchurch
25C. Cloud morning and night, otherwise fine. Northeast developing morning.

Dunedin
27C. Fine apart from cloud morning and night. Northeast winds.