If the damage to your lot is due to a defect or defects in the common property, and from what you have said it sounds like that is the case, then the Owners Corporation is responsible for the cost of repairing the damage.
To that extent it is irrelevant whether the things in your lot form part of the common property, or whether strata insurance covers it. Even if their insurance doesn’t cover it they are responsible.
For example:
– leak in roof which is common property causing damage to inside of unit – OC responsibility to fix damage.
– a fire caused by a fault in something that forms part of your lot, e.g. wiring installed by you, causing damage – your responsibility.