On the assumption that the vote to create the Special By-Law (SBL) was by a minimum of 75% of those present at the AGM being in favour, and it’s been since Registered with NSW Land & Property Information (incorporating the old Land Titles Office), then the only means to amend it is by way of another 75% vote in favour (of doing that) at another General Meeting provided the current beneficiaries of it agree in writing to that amendment.
Conversely, if the SBL didn’t achieve that minimum 75% majority, and/or it’s not Registered, then it doesn’t exist and those two (2) Owners have illegally changed and/or added to the Common Property and unless they agree to reasonable usage terms by other Owners, your Owners Corporation (O/C) could require them to remove their lifts.
Sounds to me like your O/C may have obtained poor legal advice in the first instance.