Appointment of a compulsory strata manager probably won’t work and is possibly an overkill
The offer of the OC of a dye test by the plumber is a reasonable course of action. Remember that you hired the builder. The saying he who pays the piper calls the tune is applicable here. The OC just wants to get an independent verification of the cause.
I’d do it as it will prove (if your builder is correct) the cause of the leak.
Also if you go to NCAT(without the plumber report) , the OC is going to say but we are getting an independent test done so dont make an order yet
Once they get the plumbers report insist they make a plan to fix the problem and if none is produced in 2 months then go to NCAT and seek orders to repair the common property
$400-500k seems a lot of money to repair a membrane on a single balcony. I’d be assisting the OC by getting some other quotes (the new act says multiple quotes are required for work about $30k)