Southerly is the name of a storm or front of air coming from the south. In the Southern Hemisphere these can be cold and have bad weather. In Wellington, New Zealand these storms are normally short and frequently have winds gusting between 120 km/h and 160 km/h though higher speeds are known. In Sydney, NSW, Australia these events are known as southerly busters.
In the hot Sydney summer, the land heats up rapidly each morning, and faster than the sea. The prevailing wind is a light south west breeze that blows from land to sea, but by mid to late afternoon, the land rapidly cools and a north east convection current wind sweeps in from sea to land. This is a typical "Offshore Sea breeze", but often it is strong offshore gale bringing a rapid fall in temperature, and a short, violent rain/hail storm.
A more persistent but potentially violent Sydney NE storm is known as a "Black Nor'Easter" [The "Easter" refers to the compass point, not to the religious festival]. This is not a convection wind, but a storm system that develops offshore which can last several days. This is heralded by the rapid build-up of dense black cloud that can convert to a gale in well under one hour.
Occasionally, when there is a strong cold system to the south west, the temperature is broken by an equally violent storm-laden south west wind, and it is this that is known as a "Southerly Buster".
All these wind storms have been known to cause widespread damage onshore, and to sink small craft on the water. Yachtsmen on Sydney Harbour in the summer know to keep looking at the flags near Garden Island Naval base for the black flag that warns of these strong winds, to listen to the Marine Weather Channel 67 VHF and to watch for the "Roll Clouds" that precede the cold front ahead of a cold southerly change.