In the Australasian region there is no semi-permanent anticyclone, as exists in subtropical latitudes in the Indian and eastern Pacific Oceans. Instead, a continual eastward migration of anticyclones takes place, roughly at weekly intervals. Most of the centres pass over or to the north of the North Island. The low-pressure troughs which separate successive anticyclones are associated with deep depressions centred far to the south. A period of disturbed weather accompanies the trough, with a change to cold southerly or south-westerly winds as it advances north-eastwards over New Zealand. Conditions improve again with the approach of the next anticyclone from the west. While this simple progression dominates the day-to-day weather, the situation frequently becomes much more complex. The troughs are unstable systems where depressions commonly form, some of which develop into vigorous storms that travel south-eastwards across New Zealand.
The anticyclones themselves continually vary in size, intensity, and rate of movement. Their tracks are furthest north in the spring, on the average, and reach their southern limit in late summer or early autumn when most of the centres cross central or southern New Zealand. At this time of the year, too, northern and eastern districts of the North Island occasionally come under the influence of deep cyclones of tropical origin.