Niwa's climate outlook for November to January says a very warm start to November is expected.

Temperatures could be climbing into the 30s later this week in some eastern parts of the country, for the first time since last summer.

"We have a warm northwest flow coming off the Australian continent, and the wind is a foehn effect," Niwa principal scientist -forecasting Chris Brandolino said. A foehn effect was where a mass of air dropped its moisture and warmed as it passed over a landmass, like a range of mountains.

"The warm air mass is being warmed further by the foehn."

Mytchall Bransgrove/file pic Beach weather is coming to some eastern areas, with temperatures possibly topping 30C on Wednesday or Thursday.

Places in the east of the South Island could be flirting with or exceeding 30 degrees Celsius on Wednesday. The same could happen in Hawke's Bay on Thursday. "It's certainly on the table."

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It might not happen in a large population centre, but he would be surprised if it didn't happen somewhere, Brandolino said.

The 30C mark was almost cracked on Saturday at Niwa's office in Riccarton, Christchurch, where the temperature peaked at 29.9C.

"I will be very shocked if we don't at least have a very similar temperature," Brandolino said.

Dunedin also got to 25.6C on Saturday. That temperature and the one in Christchurch were the warmest since mid-February, Brandolino said.

Conditions on Saturday were similar to those expected later this week, with a warm air mass, and a foehn wind coming off the Southern Alps.

MetService was on the same page as Brandolino when it came to the temperatures in Hawke's Bay, predicting 29C in Hastings on Thursday - after 27C on Wednesday - and highs just 1C lower in Napier.

But MetService was not expecting similar highs for the South Island, with Christchurch forecast to hit 25C on Wednesday and 24C expected for Timaru and Blenheim.

Brandolino expected the South Island to cool down more by the end of the week, than does the North Island.

"The air will turn cooler behind this warm surge for late in the week, but for the North Island the warmth does hold on - maybe not flirting with 30, but it will be above average for the rest of the week," he said.

"As the South Island goes through a bit of a changeable pattern, a bit of chilly or cooler than normal air mass moves over after this warm spell. The North Island will not experience that coolness."

Meanwhile, Niwa warned a band of heavy rain could bring flooding to the West Coast and the top of the South Island on Wednesday and Thursday.

It would be from a plume of moisture in the Indian Ocean that was heading towards New Zealand, Niwa said.

By early next week, Brandolino expected the South Island to be turning warmer again, with temperatures likely to go above average, although they may not be as warm as he was expecting this week.

It was premature to say when the next period of weather over 30C might be.

"We feel pretty confident this upcoming next few days, that's when 30-plus degree weather is going to be possible," Brandolino said.

"As we look into next week, we will have to see how that goes."