That did not discount the threat of rain on match days. Niwa expected normal levels of rain - around 91mm in Auckland - to fall this month.
Auckland, the base for most of the World Cup's knockout rounds, usually has 16 rainy days in October. But that rainfall can be as little as 0.1mm and can pass quickly. Wellington, which will hold two quarter-finals, usually has 15 days of rain in October.
Aucklanders can expect temperatures of between 10C and 19C for the rest of the World Cup.
Dr Mullan said a new La Nina cycle was building in the Pacific Ocean, which would mean a warmer start to summer. Auckland is expected to have temperatures in the mid-20s by December. Last summer's La Nina cycle was one of the strongest in 30 years, and led to heat records and floods in New Zealand and Australia.
Niwa said it was too early to estimate the intensity of this cycle.
The Southern Oscillation Index, a key indicator of a La Nina/El Nino cycle, is currently +1. The further away from zero the reading, the more abnormal it is. Anything above two, positive or negative, is considered extreme.
Niwa also reported that September was drier and cooler than usual, with the least rainfall on record in Whakatane and half the normal levels in the Bay of Plenty and Gisborne. The minimal rainfall meant soil moisture levels were already low in Canterbury, Central Otago and parts of the North Island.
September extremes
* Highest temperature: 22.3C, Hanmer Forest, Sept 30
* Lowest temperature: -6.1C, Pukaki Aerodrome, Sept 14
* Highest one-day rainfall: 98mm, Milford Sound, Sept 10
* Strongest gust of wind: 126km/h at Castlepoint, Otago, Sept 1
Spring forecast
* Warmer than usual, normal rainfall, southwesterly and westerly winds
Main centres in October
* Auckland: 16 days of rain, 10.5C to 17.8C
* Wellington: 15 days of rain, 9C to 15C
