Wait on a moment.

Much of the info above is not ALWAYS strictly applicable. It depends on WHERE you originate in many cases.

I just booked a ticket to "New York" from here in SYD.

I am able to have MY CHOICE of flying into Newark and out of JFK etc, and originally did just that as it suited my other flights on other PNRs.

As far as UA is concerned JFK/EWR/LGA are "NYC" equal co-terminals for the purposes of an INTERNATIONAL booking - from here at least on every fare I have ever seen rules for.