[dave295]

Thoughts and ideas for you Tony.
Grab a couple or three shipping containers, either/and 20 or 40 foot.
Weld them to footings, with a gap between them.
Attach some beams across the gap, throw some Z or C purlins across them and screw on a metal roof (corrac, trim deck etc.) and then concrete the gap (say 4-6 metres).
Close in the ends with a wall and/or doors and cut any other doors and windows you want in the containers.

It's done up here in the Territory and if done right it can be cyclone proof.

On a separate note putting a toilet in your shed may impact on Council rules as it may then become a dwelling. It might be worth checking.

Dave