Summer in Europe and winter in New Zealand occur at the same time because of the tilt of the Earth's axis and its orbit around the sun.
During the northern hemisphere summer, the Earth's axis is tilted towards the sun, causing the northern hemisphere to receive more direct sunlight and experience warmer temperatures. At the same time, the southern hemisphere is tilted away from the sun, receiving less direct sunlight and experiencing cooler temperatures, which results in winter.
Conversely, during the northern hemisphere winter, the Earth's axis is tilted away from the sun, causing the southern hemisphere to receive more direct sunlight and experience warmer temperatures (summer), while the northern hemisphere receives less direct sunlight and experiences cooler temperatures (winter).
This phenomenon is known as the Earth's axial tilt and is responsible for the seasonal variations in temperature and daylight hours experienced around the world.