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The combination of northwesterly winds and the foehn effect which hit the South Island at the start of August combined to produce record-breaking temperatures, Niwa says.
In its August climate summary, Niwa details how the warm weather hit at the end of July and continued into the first few days of August.
As a result, 13 locations around New Zealand recorded their warmest August daily maximum temperature on the first day of the month, including Dunedin (21.7degC) and Balclutha (20.9degC).
Queenstown recorded its second-highest temperature of 18.9degC, Ranfurly its second-highest (18.4degC) and Alexandra its fourth-highest (20.3degC).
The conditions also meant 37 places around New Zealand recorded their warmest minimum temperature, including Ranfurly (11.9degC).
The second-highest minimum temperature was recorded by Dunedin (13.3degC) and Lauder (12.2degC) and the equal second-highest by Wanaka (9.4degC), Alexandra (11.6degC) and Nugget Point (10.5degC).
Along with the warm conditions came strong winds. Record gusts were recorded at Tara Hills (98kmh), Wanaka (93kmh) and equal fourth-highest in Queenstown (85kmh).
By contrast, Middlemarch recorded the lowest temperature of the month, -7degC on August 4.
The climate statistics for New Zealand's six main centres showed Auckland to be the warmest, wettest and cloudiest, Christchurch the coolest and driest and Tauranga the sunniest.
Dunedin failed to rate, even though its rainfall was only 76% of normal at 43mm and its mean temperature was -0.2degC lower than normal at 7.6degC.
The foehn effect happens when moist air flows up and over a hill or mountain, creating higher temperatures as it descends on the downwind side.