Your assessment seems reasonable to me. On the face of it, a defect of the common property and delay in repairing it caused damage to your property. I suggest pushing to be given reasons why an insurance claim was rejected. If it was to do with the OC’s delay taking action, that is good for you.
If that does not shed any light, I think an ACAT application could be a reasonable course of action. The Tribunal is set up to be user-friendly and inexpensive. Parties are generally self-represented and costs are generally not awarded. An initial ‘directions hearing’ will be a mediated conversation between parties to hear what each side’s position is. At that point the Tribunal will try to steer the parties to a mutually agreed settlement or might send you away to discuss further or gather further information. The Tribunal member will not tell you what you should do and will avoid expressing an opinion on whether any proposal is reasonable or not. If you do agree to some settlement, that will be made ‘a consent order’ and both parties are then obliged to stick with and carry out whatever was agreed. If you can’t agree, then the matter may be listed for a hearing.
Your application should be clear about what you want the orders to be an include evidence such as a quote for the cost of repairs and a time-line of dates such as when the leak was first noticed, when you first notified the OC, what responses you got etc.
It would be worth reading through previous ACAT Unit Titles decisions and finding one that is similar to your circumstances. Then you can refer to the previous reasoning. Look here http://web.ocnact.org.au/decisions-by-acat-test and here https://www.acat.act.gov.au
A hearing will be more formal and at the end you will be given orders according to the views of the Tribunal member, which might not please either party.
Having been to the Tribunal several times now representing our OC, it is not as scary as it first seemed and the Tribunal members have generally been impressively patient and fair.