Tessa van den Dijssel 4, splashes in the paddling pool at the Esplanade.

Ice creams, swims and shade are in order in Manawatu as two ingredients combine to push the mercury above 30 degrees Celsius in the region.

MetService forecast temperatures will reach 31C in the region on Monday, one degree higher than its Sunday prediction.

Meteorologist Claire Flynn said that was not normal for the area, and occurred because of something called the "foehn effect".

David Unwin/ Fairfax NZ. Families cool off down at the Esplanade paddling pool after a heat wave hits Palmerston North.

Wind from the south-east was blowing up the Wairarapa side of the ranges, then heading down into Manawatu.

As the wind headed down the ranges, it warmed up, raising temperatures wherever it went, Flynn said.

The foehn winds, when paired with the warm air mass sitting over much of the North Island, combined to raise the temperatures.

Flynn said the opposite usually happened, with cooler north-westerly breezes going across Manawatu, up the ranges, then down into the Wairarapa and Hawke's Bay regions.

That was exactly what would happen on Tuesday, when a low pressure system moved across the country, she said.

It would bring rain, but also westerly or north-westerly winds.

That wind would go up the Manawatu side of the ranges, then warm up as it went down into Wairarapa and Hawke's Bay, she said.

The low pressure system would not be cold, but would push the temperature in Manawatu down into the mid-20s.

Palmerston North residents made the most of the good weather by playing in the pool, and soaking up the free Access Manawatu Sounds of Summer concert in the Esplanade on Sunday.

Access Manawatu manager Fraser Greig said the weather had helped draw a record crowd of more than 600 people.

"Last year we were in the 500s, but this is the first time we have been over 600.

"Hopefully it will build for the rest of the series."

The concerts were sponsored by the Palmerston North City Council and Destination Manawatu, and were held each Sunday during summer, so it made sense for ratepayers to turn out in force and get their money's worth, he said.