The four regions are as follows: 
REGION A: “Normal”. Wind speeds of up to 155 km/h 
REGION B: “Intermediate”. Wind speeds of up to 188 km/h 
REGION C: “Cyclonic”. Wind speeds of up to 220 km/h 
REGION D: “Severe cyclonic”. Wind speeds of up to 260 km/h 

Terrain  Categories

Terrain categories are another important factor in building design and engineering. They relate to an area’s exposure to wind as a result of the surrounding terrain.  

TERRAIN CATEGORY 1:
Exposed, isolated, flat, treeless and poorly grassed open terrain with few or no obstructions, measuring 10km radius or more. 

TERRAIN CATEGORY 2:
Open terrain including grassland, with well-scattered obstructions of heights generally between 1.5-5m, such as trees and buildings, but no more than two obstructions per hectare. 

TERRAIN CATEOGORY 3:
Terrain with numerous, closely-spaced obstructions, usually between 3 and 10 metres high. This terrain category usually relates to suburban, built-up or densely wooded areas. 

Importance Levels

“Importance Level” relates to the level of consequences to people or the public in the event of a building failure, as designated by the Building Codes of Australia. (BCA) There are four importance levels, as follows: 

IMPORTANCE LEVEL 1:
Buildings with a low degree of hazard to human life and other property in the event of a building failure. Typically, level 1 should only be designated to isolated storage buildings. 

IMPORTANCE LEVEL 2:
The default or ordinary level, for structures with medium consequence to human life and considerable economic consequences in the event of failure. 

IMPORTANCE LEVEL 3:
High consequence for loss of human life or very great economic, social or environmental consequences in the event of failure.  

IMPORTANCE LEVEL 4:
Exceptional circumstances, for buildings essential to post disaster recovery or those associated with hazardous facilities. A hospital, for example, would be considered importance level 4.