"Such changeable weather is typical of spring," Niwa meteorologist Tristan Meyers said.
The month was characterised by a westerly flow anomaly across most of New Zealand, which brought several strong frontal systems and periods of wet and windy weather.
"One such front brought gusts in excess of 140kmh to parts of the South Island and North Island during mid-September.
"However, these unsettled periods were interspersed by dry and calm weather."
Small areas of above-average temperatures were observed in the Coromandel, eastern coastal Auckland, and the Canterbury Plains, but "discontinuous pockets" of below-average temperatures occurred in parts of Waikato, Bay of Plenty, Nelson, Otago and Southland.
He said a near-record daily maximum air temperature for September, of 21.2dgeC, was recorded at Franz Josef Village on September 23, the second-highest September temperature on record since 1953.
Near-record mean minimum air temperatures for September were recorded at Middlemarch (third-lowest on record) -1.3degC and Dunedin Airport (fourth-lowest on record) -1.2degC.
"Overall, the nationwide average temperature in September 2021 was 10.7degC. This was 0.2degC above the 1981-2010 September average."
Mr Meyers said above-normal or well-above-normal rainfall was observed across most of Northland, Auckland, northern Waikato, Bay of Plenty, eastern and southern Hawke’s Bay, parts of Taranaki, Manawatu-Whanganui, Wellington, Tasman, Nelson, Marlborough, along the Southern Alps, and much of Otago and Southland.
Rainfall was below normal around the Bay of Islands, southern Waikato, northern Taranaki, the Kapiti Coast, and much of the Canterbury Plains.
Near-record September rainfall totals were recorded at Manapouri’s West Arm Jetty, where 625mm fell (the fourth-highest level since 1971) and Tiwai Point, where 161mm fell (the fourth-highest since 1970).
Tiwai Point recorded its second-highest one-day rainfall, when 35mm fell on September 9.
The highest wind gust of September was 222kmh at Cape Turnagain on the 10th.
Near-record gusts were recorded at Middlemarch (120kmh, second highest on record) on September 12, Clyde (98kmh, second-highest) on September 9, South West Cape (172kmh, third-equal highest) on September 4, and Oamaru (104kmh, third-highest) on September 10.
"The Canterbury and Otago regions were among the worst affected areas, as small buildings were damaged, roads were blocked by fallen trees, and almost 7000 homes were left without power," he said.
