I would ask the Owners Corporation to
1. Supply the section of the act which authorizes the ability to pass on the costs.
Basically they can’t add costs to your lot account.
If you did not fix the leak after they told you to (and you did), then they can pass on the costs of the works to maintain/limit damage to common property. That is in the Act.
Call their bluff.
Seek an application for mediation with NCAT, and/or get a Lawyer; SGL are very good at it. To inform them that they have no rights to forward the costs and the amount including interst and other associated charges are to be removed.