This is more complicated than it seems. Your main issue is with the owner of the apartment that was the source of the leak, their issue is with their builder. I would be keeping the strata committee onside in this as this is not a common property failure but you can get them to help you confirm that the other apartment was at fault.
Get an estimate for the repairs ASAP and send it to the owners in the apartment above. Then they can claim it against their builder and the builder can claim it against their insurance.
However, just to complicate matters, your “fixtures and fittings” may well be covered by strata insurance so I would contact the strata insurer directly to see if this is the case. You might get push-back from the strata committee on this as a claim could affect future premiums.
If that is the case, it would be in the OC’s interests to pursue the apartment owner and builder. Did I say it was complicated?