We booked an airline ticket with STA travel for our son's trip to Australia on October 5, 2015. The travel agency was recommended by the Australian outfit he is on the trip with. All flights confirmed received paperwork. We go to Newark airport to drop him at first flight to San Francisco with United on Jan 25, 2016 and were told that his ticket had been cancelled. Apparently Air New Zealand who tickets were purchased from failed to confirm flight with United within 24 hours of the booking and our son's ticket to San Francisco and Sydney Australia were both cancelled on Oct 8, 2015 No one ever notified us of the cancellation which occurred according to United on Oct 8th 2015. We received travel confirmation of RT flights on October 8th from STA Travel. STA agency - said they had never run into this situation before and said it was a "glitch" in system. We have gotten the run around from just about everyone involved.
Well the first question would be, how did your son get to Australia? Did United, STA and/or ANZ sort out a flight for him and you are simply wondering who to seek some compensation from for the hassle? Or did you have to buy a new last minute ticket?
If you had to buy a new ticket, then STA is the one you need to work with to get the previous ticket refunded. They are the agency of record and all payments go both ways through them (especially since it may well have been a consolidator ticket, as STA is a huge ticketing group). Ultimately they have to get the funds back from whichever airline is the marketing carrier and may delay the refund using that excuse, but STA should be big enough and professional enough to pony up the refund before they finalize with ANZ.
If his flights were sorted out without any additional ticket purchase and all you want is some compensation for the hassle or delay if any, I don't want to be pessimistic but good luck. A glitch in the system, a lost data packet or two between computers happens and it is hard to prove blame for anyone party. Your best bet in this scenario is to write the airlines involved a polite, factual letter outlining the issues and hope you reach a caring ear. You may get something like extra frequent flyer miles, maybe get real lucky with a voucher towards a future trip, but don't count on any major compensation.