Do you still have the documents relevant to your purchase of the property? A look at the plan in the Contract for Sale should inform you whether or not the roof forms part of your property or is common property. Or, ask the Strata Manager (if you have one) to explain the way levies are calculated – levies are worked out on the portion of the whole block of units that you own, and this may shed light on how much you own – so if the floor area of your unit is the same floor area of the unit below, but your levy is calculated as say 17% of the whole and your downstairs neighbour’s levy is 13%, then clearly you are paying for more because you own more – and the more you own might be the roof.
In my experience it would be very unusual for a unit owner to own the roof above their unit – after all, it isn’t just the roof for your unit, it’s the roof for the whole block.
The fact that the Owners Corporation used the Sinking Fund to do works on the roof the last time suggests that it is Common Property.
But either it is Common Property – and therefore the responsibility of the Owners Corporation – or it isn’t. Get a plan of your building (from your purchase document or ask your Strata Manager) and work it out.
If it fills with water and this is likely to cause damage to your unit, it may be in your best interest to pay for repairs to the roof before things get worse – and maybe sort out liability later.
I can’t see how undertaking work on your “unfenced” roof would be any more dangerous for a properly qualified roofing contractor than working on any other roof. But unless you have qualifications for working at heights, I’d suggest you keep off the roof.