If you have a record that you have asked the question and it has been rebuffed or ignored, I would initiate mediation proceedings at Fair Trading with a view to taking the strata committee to NCAT seeking orders under Section 232(2) Failure to exercise a function (below).
I would also offer some solutions, such as an audit or change of code for garage keys, the installation of lockable bollards and a request to investigate and cost alternative arrangements.
If you install bollards and residents each have a universal key, then only genuine visitors can park there when residents are at home. You might also need security cameras to monitor the spaces to make sure residents are doing the right thing and locking up when their visitors have left.
Regarding mediation, it’s free and just making an application might be enough of a warning shot to get things moving.
Section 232 (2)
Failure to exercise a function
For the purposes of this section, an owners corporation, strata committee or building management committee is taken not to have exercised a function if—
(a) it decides not to exercise the function, or
(b) application is made to it to exercise the function and it fails for 2 months after the making of the application to exercise the function in accordance with the application or to inform the applicant that it has decided not to exercise the function in accordance with the application.