It is the time at which the sun is at its northernmost point in the sky due to the tilt of the earth's axis. At noon the sun reaches its lowest altitude.
As the sun faces the most northern point here in New Zealand, we will receive less heat, so that becomes the Winter Solstice, the shortest day.
Winter Solstice is the darkest time, it is not the coldest. From now on the days will get gradually longer but the worst of winter is still yet to come. It takes at least three to six weeks after Winter Solstice for the coldest air temperatures for the year to occur, due to the oceans ability to store and release heat.
As the days get longer . . . the cold grows stronger.
Now the days will get longer, by minutes at first before speeding up closer to the Spring Equinox - Ostara on September 23, when the sun is over the equator.