The question is will entering SYD into a search include SYD+WSI, like say it does when putting SYD-MEL into ExpertFlyer where it brings up MEL+AVV? I assume it will.
This happens because the search recognises IATA city codes, which typically refer to multiple location (airport) codes in the same city. The IATA city codes can be distinct from airports (LON, NYC, TYO, etc.) or like the case you describe, can be the same as the major airport in that city. MEL as a city code includes the airport codes MEL, MEB (Essendon), and AVV - since MEB does have some airline services* I'd expect it should show up too, but it's possible those aren't published in the GDS's and thus not relevant to ExpertFlyer? SYD is also I believe an existing city code with SYD and BWU (Bankstown), but as there are no airline services currently at Bankstown, it won't have any effect in search engines.
In this case, I'd expect WSI to be added to the SYD city code. What the threshold is for establishing a distinct city code that isn't the same as the largest airport, I'm not sure. In some cases like TYO, it is the code that originally referred to the older original airport (now using HND) and was established when they built a second airport (NRT) and they didn't want to give one precedence over the other. With JKT for Jakarta, the original JKT airport closed down after CGK was built, and JKT was retired as a location code but there is also HLP airport, so the city code remains in order to refer to both CGK and HLP. Some city codes like NYC and CHI appear to go back to the very early days of the IATA codes, since these cities already had multiple major airports when the system was introduced. I believe BJS for Beijing is a newer city code (with no airport called BJS) but I don't know its history, as I'm pretty sure it existed before PKX opened.
* Link Airlines flies from MEB to DBO, OAG, and WOL, while Sharp Airlines flies MEB to GFF, KNS, and FLS. Maybe try a search MEL-DBO to see what happens?
** Full list of currently useable city codes here:
IATA airport code - Wikipedia (as I said, I think SYD is technically one as well, but not listed here since it has no effect with BWU not having any airline services any more).
