There are different wind regions and wind speeds around Australia. Generally speaking, all areas lower down in latitude than Bundaberg QLD are in the same wind region. From there, the topography and proximity to the open ocean are all factors which affect the expected wind speed in an area.
If your fence is in far north Qld or at a similar latitude, you should seek advice from an engineer on the suitability of a colorbond fence at all, even without extending the height.
If you are south of Bundaberg, you need to consider the factors that may affect wind pressure in your area. The main factor is shielding from the wind by houses, trees, sheds etc. In most suburban areas, there is sufficient shielding for the fence to be well able to be extended safely. In any case, the connection of the Screenline Extension to your fence is designed to withstand any wind, but you need to be sure your original fence is sturdy and has been properly installed. This being the case, 90% of fences will be suitable to be extended.