In another post we talked about parking allocation being part of a DA process and is dictated by council (usually to a lot number/bedroom and use formula): even if the OC wished to engage your point of view they would need to seek approval from council. Additionally, if the car parking is underground, the BCA then requires that the car park be adequately ventilated: that one extra car could infact cause significants costs if mechanical ventilation needs to be upgraded or installed. (The BCA also uses a formula to dictate what is needed). There is also the impact to your buildings’ insurance and even fire certificate. For your OC to consider your request to ratify a misuse of parking, they need to cover themselves and that involves council searches and consultants(BCA and fire), none of which is free: irrespective of a financial offer, if they reject (either because they want to or have to) then the OC is still out of pocket merely by engaging council/consultants and the like.
You advised that you are in Sydney City, and, even though your scheme dates from the 70’s, Sydney council has an active policy of discouraging car spaces, so is unlikely to retrospectively approve 3 car spaces for a lot that has 2 car spaces.
All of that aside, it appears that you parked first, and asked later, which is probably not the best way of going about things. You may have had more had more luck in getting the permission had the OC not had cause to instigate action.
Granted, from what you describe, the OC have ignored the opportunity to enter into a dialogue of compromise but then again they actually aren’t obliged to: common property is not terra nullius to be claimed, no matter how innocuous the action of misuse or encroachment.