Hi Mailbox
Are you in NSW?
It’s hard to tell if you don’t have the original plan drawings. Check your original sale contract or pre buy strata report to see if there’s a copy in there.
I know for my 1960s building I had to look at both the plan and the certificate of title. My certificate specified that Strata Schemes (Freehold Development) Act 1973 Schedule 4 Clause 3 applied to boundaries, which moved most boundaries from the centreline to upper, lower and inner surfaces. Except where marked with a note on the plan. My plan has notes specifying that lot property of garages extends to the outer face, and so includes the garage doors.
The SCA Who is responsible? guide is only a guide, as pre 1974 buildings are a bit different and you do need to look at your certificate of title and the plan.
Maybe the other owners have a copy of the plans they can show you? Common property is generally the thick line, and lot property the thin lines. And then any notes will specify further.