Map from Google - Pitcairn_Islands

The climate of the Pitcairn Islands, a British territory in the Pacific Ocean, just south of the Tropic of Capricorn (therefore in the Southern Hemisphere), is subtropical oceanic, with a hot summer from December to March, in which the maximum temperatures are around 27/28 °C (81/82 °F), and a very mild winter, with maximum temperatures around 22/23 °C (72/73 °F) from June to September.
Here are the average temperatures of Adamstown.
Average temperatures - Adamstown
AdamstownJanFebMarAprMayJunJulAugSepOctNovDec
AdamstownJFMAMJJASOND
Min (°C)222322212019181818181921
Max (°C)282828262523222223242627
Min (°F)727372706866646464646670
Max (°F)828282797773727273757981

The rains are frequent in all seasons, so much so that in a typical year, 1,600 millimetres (63 inches) of rain fall, and in all the months precipitation exceeds on average 100 mm (4 in). Here is the average precipitation.
Average precipitation - Adamstown
AdamstownJanFebMarAprMayJunJulAugSepOctNovDecYear
AdamstownJFMAMJJASONDY
Prec. (mm)1201401201551301551501251401351201651635
Prec.(in)4.75.54.76.15.16.15.94.95.55.34.76.564.4
Days141313121111111212131414150

The wind often blows, moderate or strong, especially in winter, when weather fronts brought by the westerlies often pass at this latitude, and there are periods of bad weather.
The amount of sunshine is high in the summer months, and is still acceptable in the winter months, when there are also sunny days.
Adamstown, the capital, is located in the only inhabited island, the southernmost of the four islands of the archipelago, in which the descendants of the Bounty mutineers reside.
The Pitcairn Islands can be brushed, though rarely, by tropical cyclones, usually not intense because the waters in which they are located are not warm enough, and therefore do not provide a lot of energy. This happens from November to April.

Adamstown

The sea temperature is high enough in summer, while from July to October it becomes a little cool for swimming.
Sea temperature - Adamstown
AdamstownJanFebMarAprMayJunJulAugSepOctNovDec
AdamstownJFMAMJJASOND
Sea (°C)262626252423222121222324
Sea (°F)797979777573727070727375

When to go

The best time to visit the Pitcairn Islands runs from January to March, being the warmest and sunniest, and also the best for swimming and sunbathing. Sometimes, however, in this period tropical cyclones may pass, or rather their remains. You can therefore come in the other months, particularly in September and October, when, however, the weather is a bit cool for swimming.

What to pack

In winter (June to August): spring/autumn clothes, light for the day, sweatshirt or sweater, jacket, raincoat or umbrella.
In summer (December to February): light clothes for the day, a scarf for the breeze, a sweatshirt for the evening, light raincoat or umbrella.