A Skillion (flat) roof offers a lower wind profile, and is generally lighter in weight to a gable roof. My roll off skillion roof has withstood sustained winds of over 100kph without any damage, and I live only a couple of hundred metres from the coast.. Of course any roll off roof is only as good as the method used to secure it to its track, and the skirting and sealing of the area where the roof and walls meet. Some form of wall bracing, independent of the roof (like corner joint 45 degree braces), and the use of galvanised wall panel braces to prevent racking, should be a given for your design. Consider the foundation and how you attach the observatory to the foundation. The Qld standards require anchor points top to bottom, so obviously the wall bottom plate has to be bolted to the floor structure or slab (if used), and the roof must have rated tie downs which extend and anchor below your roof track. Honestly C2 seems extreme for a small out building, without habitation, but if that is the mandate so be it. I would recommend you forgo any windows, mine does not have any, and it enhances wall structure and strength, and there is no glass to break and provide an entry point for the the wind. Finally, consider the observatory orientation, and the usual cyclonic wind direction in your area; position the building with its narrowest profile towards the expected wind direction, and take advantage of any natural land features that can shelter the building, like a low hill, other rated buildings, etc. Check on your local Council policy on requirements for garden sheds, many Councils will have much less stringent standards for these. Note that if you keep the size to 10 square metres, with a wall height of 2.4m, you can likely escape regulation, see this link:
https://www.cheapsheds.com.au/queens...uncil-approval
Good luck, let us know what you come up with.