The Rebbe on the Need for Local Knowledge and an Expert Rav
The Rebbe treated a city eruv as a matter of detailed local halacha. General scholarship or a description from a distance could not replace examination by a Rav expert in the subject.
A Rav with special expertise in hilchos eruvin
In his letter to Rabbi Pinchas Weberman (3 Tammuz 5722), the Rebbe directed that the question be brought to a Rav with special expertise in hilchos eruvin. These halachos require mastery of difficult sugyos together with the ability to apply them to streets, structures, and patterns of public use.
A respected Rav may be a posek in many areas and still defer a city eruv to someone with particular experience in the subject. Public reliance demands that level of competence.
A full investigation of local conditions
The Melbourne response applies the same approach. The Rebbe called for qualified Rabbonim to conduct a full investigation of the local circumstances before the proposal could be handled publicly.
A map and a list of apparent walls cannot answer every question. Roads may pass through a boundary; bridges and rail lines may create openings; legal ownership and sechiras reshus may be incomplete; and the status of the city streets may affect whether tzuras hapesach can work at all.
The city itself must be known
Population, traffic, street width, whether major roads are mefulash, walls, gaps, public access, the proposed route, communal arrangements, and regular inspection all belong to the local inquiry.
This is why an eruv accepted in one Chabad community cannot decide the status of another. Crown Heights must be judged as Crown Heights, by the Rabbonim responsible for it.