Yes you’re right, based on what you have said there are five owners who have lots that include garage spaces in the underground garage. It isn’t clear what part of the underground garage may be common property, but to the extent that it may be, even if there were space to install stack parking in the common property for use by other owners, that would require a special resolution of the owners corporation because you would be making a change to the common property.
If it affected the use by you and the other 4 owners of your parking spaces, then you and the other owners would have to specifically agree to that.
Also presumably Council approval was given to the property on the basis of the underground garage as it is, so any change would have to be approved by Council.