Timaru has broken the record for its hottest day ever, with the temperature hitting a scorching 40.3 degrees shortly before 4pm.

The extreme heat is connected to the same heatwave that saw Sydney climb to 42 degrees yesterday and then record the hottest night ever, according to WeatherWatch.co.nz.

The city has just gone through its longest spell of 30-degree days in 150 years of records - seven days.

Weather analyst Richard Green says a similar phenomenon occurred in 1973.

"Sydney reached 27.6 as their overnight low last night, beating their record by a whole degree set in February 1973. That same heat wave also hit Timaru in 1973 and again today that record has been broken by point six of a degree".

Other centres to melt in the heat today were Christchurch, Alexandra, Ashburton and Oamaru, which all officially reached 36 degrees.

But the weather news authority says Timaru can expect to drop 20 degrees quite suddenly later this afternoon or early evening as a southerly continues to move through.

The South Canterbury districtis home to nearly 43,000 people.

It proved too much for several elderly people, with a St John Ambulance spokesman telling NZPA they had attended several calls to people fainting from the heat.

Dunedin reached 35 degrees before the southerly while Gisborne and Blenheim reached 34.

Numerous other regions and cities were in the 30s and late 20s across the country with the coolest weather on the nations western and southern coastlines of both islands.

Auckland made it to 28 degrees with unofficial recordings reaching the low 30s across the region.

WeatherWatch.co.nz says the cooler change will move in to the North Island on Monday and reach Auckland by Tuesday night, lowering humidity levels too.