Am I liable for leaks under bath?

Got an urgent, complex issue with my investment property.

My (spa) bath /shower over bath is leaking and has caused damage to the unit below.  It happened once before 5 years ago and an insurance claim was put in by the OC for damage caused to the unit downstairs. Recent investigation pinpointed the problem to a fine crack between bath and wall. A strip of silicone will probably prevent further leaks (as long as it lasts). Simple.

BUT…. the owner below and their plumber, who I kindly granted access to when we were trying to diagnose the problem, established that there was no waterproofing under my tub. Waterproofing under the tub would probably have staved off/delayed the damage to the unit downstairs. It was also discovered, during this investigation, that the configuration of my bathroom, as well as the existence of a spa bath, differs from the plan when the building was built, so it’s being deemed ‘illegal’ by the Owners Corp.  

I DID NOT MODIFY THE BATHROOM AND I HAVE PROOF.  It was the way it was since the day I purchased it 20 years ago. The building was just 2 years old when I bought my unit, so the bathroom was most likely modified by the builder without the Private Certifier or Council at the time noticing the anomaly (or caring).  I highly doubt the previous owner did it because it would have caused a racket with plumbing changes etc.  Plus, who in their right mind would rip up a 12 month old bathroom and spend what would be $15K odd today just to facilitate a spa bath?

The Owners Corp are now demanding I rip up the bath, waterproof under the bath at my expense and prove them its all been done within 30 days, or else….(threats of legal action, claim for past damage compensation of rent to unit below, claiming the excess on the insurance claims and any past claims that are related to leaks below etc etc).  They’re saying it’s my problem irrespective of whether I did the mods, citing that I inherited the issue when I bought the unit.  Buyer Beware is their conclusion.

If the bathroom was original and didn’t differ from the plans lodged, then I have a case  (I think), because the floor and waterproofing membrane (or lack thereof) is common property….I think.

Anyway long story short, am I liable and do I need to fix this completely at my expense? Is the fact that the bathroom config and fixtures is different to the council plan, my inherited problem and my ‘Caveat Emptor’?

Have been losing a lot of sleep over the issue since receiving the letter.  The OC are refusing to accept any responsibility because according to them, and since the bathroom differs from the plan, it must have been modified illegally and therefore, its not common property and my problem.  

What should I do next?  If I take this to the Tribunal, will they decide in my favour?  Or am I in trouble?

Thanks