EC has given away vistors’ car space

I'm not a Strata Lawyer but rather the Secretary in a Self-Managed Plan, but on the basis of my knowledge of relevant Legislation, NO your EC can't do that even with the benefit of an Exclusive Use By-Law that's been properly resolved at a General Meeting of the Owners Corporation; which clearly they don't have anyway.

Further, the visitors' parking space would have been a requirement of the Local Council's Approval of your Building's Development, and I think that as the visitors' parking space is also shown on your Strata Plan, that any assignment of that space to individal Lot Owners via an Exclusive Use By-Law would require a Strata Sub-Division through the NSW Land & Property Management Authority (the old Land Titles Office).

What to do about it ?  On the basis of my past experiences with the CTTT and Owners' Fiefdoms, I'd avoid the CTTT route at all costs, and further, if you get “heavy” with your EC they'll likely close ranks and ignore you, and that will limit your options to resolve your problem.

I would however politely advise your EC in writing that through their decision they have placed the Owners Corporation (that's you too) in Breach of the NSW Strata Schemes Management Act and the NSW Real Property Act, and that they should rescind their decision immediately and, if they wish to then properly formalise what they have done, they should convene an Extraodinary Meeting of the Owners Corporation to do so.

If / when that Meeting is convened, you will have hopefully knowledge gained via the legally qualified members of this Forum to knock it off!