Here are two definitions of double-glazing from reliable sources: Build.com.au and Action Glass (among other commerial suppliers).
What your strata committee is referring to is Secondary Glazing which is fitted on top of existing windows. I would challenge them to prove that what the Act is talking about is the more widely recognised version of double glazing and not the admittedly frequent misuse of the term for commercial purposes.
So you are trying to do something that is less significant than what the Act allows. And the way you challenge them is to request mediation at Fair Trading where, hopefully, someone will explain what the Act actually means.