Neighbours dug cellars under flats

Hoping that I can get some advice on how to resolve the alterations to our common property.  Have discovered that two owners have cut holes in the floor boards and have constructed in the foundations of the building, 2 store rooms with out approval.

Both owners are now seeking a special resolution to use this space, however very little information is available on common property under the building. They have built cellars/store rooms underneath their unit, by removing soil and constructing walls to enclose the area.

Does any one know what the building code/regulation is for NSW for the construction of an under ground cellar or store room on common property in a unit complex.