Map from Google - Cook_Islands

The climate of the Cook Islands, an archipelago in the Pacific Ocean, "free association" with New Zealand, is tropical, with a rainy period from December to March, and a relatively dry period from June to September, without, however, a real dry season.
The islands are divided into two groups: the northern islands, closer to the equator, are warmer and have a stable temperature, around 81/82 °F as daily average throughout the year.
Here are the average temperatures of the northern island of Manihiki.
Average temperatures - Manihiki
Min (°F)777577777775757575757575
Max (°F)868888888886868686888886

Even the sea temperature is high throughout the year, as can be seen in the following table.
Sea temperature - Manihiki
Sea (°F)848484848484828282848484

On the contrary, in the southern islands there is a cooler period from June to October, when the daily average drops to around 70/72 °F, while in the warmer months, from January to April, it doesn't go above 77/79 °F. In July and August, at the acme of the cool period, the maximum temperatures drop to around 77 °F in the southernmost islands of Rarotonga and Mangaia, and around 81 °F in Palmerston and Aitutaki, although the minima remains in both cases around 68 °F. However, from June to September, sometimes the temperature at night can drop to around 59 °F in Rarotonga and Mangaia.
Here are the average temperatures of the capital Avarua, in the southern island of Rarotonga.
Average temperatures - Avarua
Min (°F)737373727068666666687072
Max (°F)828484828179777779798182

In the southern islands, even the sea temperature decreases a bit in the winter months, although it remains high enough for swimming, as can be seen in the following table.
Sea temperature - Avarua
Sea (°F)818182817977757575757779

Rainfall is abundant throughout the country: in the northern islands it ranges from 75 to 80 inches per year, becoming progressively more abundant from east to west, while in the southern islands it hovers around 80 inches per year.
Here is the average precipitation in Manihiki.
Average precipitation - Manihiki
Prec. (in)1310.

The reason why in the northern islands the differences in rainfall are greatest, lies in the fact that those located to the east are closer to the arid zone of Kiribati: unlike in Kiribati, in the Cook Islands the rains are still plentiful, but during the years of La Niña, when the cool eastern winds which blow near the Equator from South America become more intense, the drought extends up to here, while in El Niño years, when the cool winds slow down, the sea warms up, and the frequency of cyclones increases.
In Avarua, more than 8 inches per month fall from December to February, and about 2.8/3.1 in from June to September. Here is the average precipitation.
Average precipitation - Avarua
Prec. (in)

The amount of sunshine is fairly uniform throughout the year, which means that even in the rainiest months there's no shortage of sunshine: showers are more intense and more frequent, but do not last long and do not shield too much the sun, even in this period.

The Cook Islands are situated in the area of the South Pacific tropical cyclones, which may pass from November to April (although they are more likely from December to March). The northern islands are located right in the area where cyclones form, so they are still in the early stages of their life, and then evolve migrating south, although it is possible that some of them could evolve rapidly, becoming very intense in the same area where it has just formed. An intense cyclone, formed a little earlier than the normal period, was Martin, who hit the islands on 2005, between October 20 and November 5.

Cook Islands

When to go

The best time to visit the Cook Islands runs from mid-May to mid-October on the northern islands, where you will find a muggy heat, but tempered by the trade winds; showers sometimes will be intense, but they will soon make room for the sun to shine.
As regards the southern islands, the best time is more or less the same, from mid-May to mid-October, but you will find lower temperatures, less abundant rains, though still possible, and the trade winds which may sometimes be a little cool for those who are sensitive to cold. It will be therefore useful to bring a sweater or a sweatshirt for the evening.

What to pack

In winter (June to August). Light clothes for the day, a scarf for the breeze, a sweatshirt for the evening, possibly a light jacket for the southern islands; light raincoat or umbrella.
In summer (December to February). Light clothes, of natural fibers, sun hat, a scarf for the breeze, a light sweatshirt for the evening, light raincoat or umbrella.