The following advice is from a QLD perspective. Check with your local council and confirm the Development Approval change of condition as it relates to traffic circulation. You may be able to do this by filling out an online request on the council website.
It would help if you had the DA approval year and reference number before you contact the council. Council may provide some advice on how to proceed but … don’t count on it being anything other than general.
Be prepared for Local Council not being able to have its local laws rangers/ parking wardens take action on private property. This is usually the case with visitor parking (in QLD) but it may be different with a traffic circulation issue, don’t hold your breath though. Ditto for the local police station who may not have parking obstruction as a high priority.
If the DA condition checks out, you might be able to write a Motion for the committee to consider at its next meeting. This is the case in QLD. Your motion would involve action being taken to remove the obstruction caused by inconsiderate parking which contravenes the DA approval and the obstruction by-law.
Or you could simply write them a letter asking for reasons why some residents are allowed to park on common property as though it is their own exclusive use area in contravention of DA approval and the obstruction by-law.
Along with your motion or your letter, you could include the advice from your local council with regard to the original DA and the advice from the SM with regards to the obstruction of common property by-law applying.
The point to make to the committee is that it appears the committee is picking and choosing which by-laws it wants to enforce. Which would be unfortunate as the committee has little choice but to enforce or attempt to enforce the by-laws.
When, as seems likely, the committee does not attempt to remedy the parking situation, you have three choices. Continue to park on the street, park on the common property like the others or progress your complaint to NCAT. When lodging your complaint, stick to the one issue, parking obstruction, and provide copies of any correspondence between you and the committee, local council information, the relevant page of the DA, and your by-laws.
Given the response you received at your AGM, it might be pointless going through a conciliation step (as available in QLD) before proceeding to an adjudication.
The fact that you’ve raised the issue at the AGM may satisfy the requirement of self-resolution of a dispute before proceeding to NCAT. (Self resolution is a principle that applies as a first step in QLD.)