The strata plan drawn when the strata scheme was created should show who the garages belong to.
If the garages are not on a lot or are not separate lots themselves, then they must be part of common property.
If they are the former, then you need to get your Unit Entitlements changed.
If they are the latter, then you need to get the current occupants to pay rent for them.
The easiest way to establish the value of the garages for either purpose is to get local estate agents to tell you how much a unit and a garage is worth (just as a ballpark figure).
So then you re-calculate the UEs accordingly. Let’s say for the sake of argument that the units are worth $500k each and the garages are worth $50k each.
Instead of your unit entitlements being 25 each, the would be 2 @ 27.5 and 2 at 22.5.
Or the owners corp could rent the garages to the current users and pass them on when that owner sells, on a first in, best dressed basis, so that access to the garage doen’t become part of the value of the flat.
Or you could even agree to sell the garages to whoever offered the most, with the money going to the sinking fund.
There are two critical issues here. First you need to establish who owns the garages. Then you need to get unanimous agreement (if humanly possible) on what to do about it.
Either way, the people currently using the garages are going to lose – the key will be to soften the blow as much as possible without cheating the other owners out of too much of what they are entitled to.
The easiest way to change unit entitlements is with unanimous agreement. But there are mechanisms via NCAT whereby a reasonable solution can be imposed by the Tribunal if one or more owners refuse.