Just when I think I have read everything …
These works are not only in breach of by-laws but are probably breaches of council planning regulations. They may also be a threat to the structure of the buildings as a whole.
Working backwards, if they want to keep their cellars, you want your neighbours to
a) guarantee the safety of the work
b) get council approval for the work
c) accept ongoing legal responsibility for the cellars and the floors, via a specially drafted by-law
d) compensate other owners for the value of the common property they have grabbed
e) compensate the owners corp for all fees and charges related to putting this right, including drafting of by-laws, building engineering checks and legal fees.
Failing any of the above, you need to get NCAT orders requiring them to reinstate the floor and underfloor area to their previous condition.
By the way, this is not a choice for the Owners Corp: you have a legal duty to look after common property and that means sorting this mess out. You don’t need to find out what the building code is – they do.
I would get very heavy, very early on this.