That force of wind happens about once a month. Its more common November, December but the last 2 years have had big winds in January, February so its a real possibility. I think if you could only pack a 4.0/ 3.7 you would be fairly safe. If you can fit it in - bring the 3.4 just in case.

The rule is for Tasmania is you will break everything and need stuff you haven't got. So just pack as much as you can. There are shops that sell windsurf gear in Tasmania but they don' stock much.