The maximum temperature of the month was on Sunday and reached 18.5C, with the average maximum across the month sitting at 15C.
Holden said apart from a few fine days, September finished off a "cold, wet, and miserable month".
MetService is predicting the rest of the week to Sunday to be mainly fine with possible showers.
The high for the first day of the month is 13C but is expected to reach 20C on Sunday. The low will drop to 3C tomorrow and sit at 9C on Sunday.
Holden said the focus on climate change had understandably taken second place to the pandemic.
"However this serious issue now remains," he said.
"To quote Sir David Attenborough's words at a recent conference, if nothing is done, we face 'irreversible damage to the natural world and the collapse of our societies'."