On the assumption that your complex is in NSW, I’m surprised that it has two unallocated carspaces available to residents on any basis let alone “first in”.
Are you sure that those carspaces weren’t originally designated for visitors?
Because if that’s the case, the provision of those would have been a Condition of the Development Consent for your complex, and your Owners Corporation cannot circumvent that except on a short-term / special needs basis.
As for the rights of residents in your complex whose occupancy isn’t covered by a Residential Tenancy Agreement, I’m afraid that their rights so far as Strata is concerned is the same as those of any other resident, but so are their obligations to comply with the By-Laws of the complex.
So in my opinion your first action should be to ascertain the original purpose of the common carspaces (the Strata Title Plan should show that), by what means those were made available for resident parking (e.g. a decision at an AGM), and whether on the basis that it may well be contrary to the approved DA and that it’s proving to be problematic, that availability should be rescinded.