The original development application for your apartment complex, which the specified number of parking spaces etc was done based upon X number of apartments with X number of bedrooms, which could then be assumed to house a maximum of X number of people. Changing the number of bedrooms in an apartment will alter this, and while if you are the only apartment in your complex to change, it would possibly set a precedent for others to follow.
Also apartment complexes are normally designed to have bedrooms over bedrooms between apartments, and living areas over other living areas. This helps with noise etc. By putting a bedroom in a living room, there may be issues down the track with noisier individuals above you, and the angst it can cause all around.
I have been in complexes where people have done unapproved installations of 2nd/3rd bedrooms in the living areas, and when building or strata management discovered this, were quick to have removed per the above reasons.
Willoughby Council is reasonably easy to deal with, so other other posters have suggested I would recommend calling them directly.