Queenstown has mountain weather, which can mean a cold outbreak, even in summer.
We recommend bringing a wind-proof and water-proof jacket and some warm clothing at any time of the year. Overnight winter temps can get as low as -8° and summer daytime temperatures can reach 25°. However there is often a cool breeze, even in Summer which can make it feel cooler. Be aware that the sun is very strong in New Zealand and a sun hat and sunscreen is strongly recommended in summer.
For more weather info see Metservice: https://www.metservice.com
