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Niwa meteorologist Ben Noll said there had been a cool start to spring, but in a couple of days, temperatures could break records and become more like those of November than September.
''For the rest of the week and over the weekend, a large area of high pressure is forecast to siphon subtropical air across the Tasman Sea on to New Zealand.''
Mr Noll said temperatures were expected to be 5degC to 10degC above average through to the end of the working week in the South Island as a foehn northwesterly airflow developed.
''Maximum temperatures may reach the low 20s in some inland areas across Otago and Canterbury.
''The unusual warmth is expected to peak on Saturday, with some towns seeing maximum temperatures potentially close to, or exceeding, 25degC.''
Otago maximum temperature records for September include 25.5degC at Balclutha in 2006; 25.4degC at Lauder in 1966; and 24.7degC at Middlemarch in 2012.
''Another very warm day is expected on Sunday before a southerly change ends the run on Monday,'' he said.
The North Island would also experience above average warmth, combined with mostly dry weather, but it was not expected to be record-breaking, he said.