The latest Island Climate Update report says the dry weather will result from strong El Nino conditions which were established last month and are expected to continue until December.
Normal or below normal rainfall and normal or above normal sea surface temperatures are forecast for the northern Cook Islands.
Sea surface temperatures for the southern Cook Islands are predicted to be normal or below normal in the El Nino period.
The total rainfall recorded for Rarotonga last month was down 47.3 millimetres from the previous month.
August produced 102.1mm of rain, while total rainfall in September was 54.8mm.
However, there was a slight increase in the average temperature last month, resulting in on average 22.8C compared to average temperature for August of 20.9C.
The highest temperature recorded last month was 28.4C, while the lowest was 16.6C.
The highest temperature recorded in August was 27.4C, and the lowest a chilly 14.4C.
“The dynamical models are all in agreement to forecast continuing El Nino conditions for the October-December 2015 period,” the report says.
“As a consequence most regions of the southwest Pacific usually affected by the South Pacific Convergence Zone are forecast to experience a drier than normal October-December season.”
International guidance indicates that El Nino conditions are virtually certain (99 per cent chance) to continue over the next three months and highly likely (95 per cent) to carry on through summer (January to March, 2016).
“Sea surface temperatures and precipitation anomalies continued to intensity in the eastern Pacific.”
The wet and hot season usually begins in November but the current El Nino conditions could delay its onset.