I know of a similar situation where leakage of lime saturated water from the roof of a common parking area (with individual carspaces forming part of the Units above) damaged the paintwork of a resident’s vehicle. The resident made a claim for damages against the Owners Corporation (O/C), their Insurers declined the Claim because the O/C was aware of the problem and yet delayed remedial works to their earlier repairs, so the O/C itself had to reimburse the resident for the costs of repairs to the vehicle’s paintwork.
Your O/C would do well to consider that outcome!
I don’t know why you would want a legal letter to formalise your use of the communal garage, or if you do, why your O/C would not be responsible for that …but as for Solicitors who could assist, perhaps speak with Makinson & d’Apice – they’re Sponsors of this Forum and one of the Firms (in NSW) who has expertise in strata and community title matters.