Thanks everyone for such quick responses!

I just googled 'minimum age top bunk' in the australian google, and a heap of links came up from websites like consumer protection depts and choice, all had recommendations that top bunk sleepers be 6 (youngest I found) - 9 years old (oldest I found) at a minimum.

I have seen actually bunk beds that can be seperated into 2 singles, so this is also perhaps an option for us....

I'm so glad to hear that it works for others - I hope it does for us too when the time comes!