To be more specific, there are actually three corridors that connect the "A" and "B" sides of the Terminal 1 which are available for use by passengers:
- On the upper level, for departing passengers, the normal passageway with travelators that you use to access gates 50-63 . There is two-way traffic here although usually it's only traffic going towards gate 50-63. You might go "backwards" down this corridor, if for example you've used the Singapore Airlines or Etihad lounges but your flight departs gates 8-36.
- On the ground level, a long enclosed corridor prior to immigration. This is only used by staff normally but can be opened for general access if immigration on one side is closed and a flight's passengers need to be directed to the other side - usually only happens later at night.
- At the baggage carousels, for arriving passengers after immigration, which is what I was referring to above. This is just a short corridor that connects both sides to the oversize luggage collection area, and normally there is free movement of passengers between the sides as well, although I suppose it could be blocked off if customs was closed on one side. If congestion at the luggage carousels means the "wrong" side is used for your flight, you might have passed through there without quite realising it, just going to collect your luggage.
Side note about the gate 50-63 area to relate back to Qantas - around 2017 I was on a SYD-JNB flight that was using that area, unusually for Qantas. Boarding was delayed at last minute for about an hour for an engineering issue, after the lounge had already announced boarding. Qantas invited all business class and lounge-eligible passengers to get a voucher for snacks/drinks to use at the airport shops in that part of the terminal, so we didn't have to walk all the way back to the QF lounges. I thought that was a nice touch and something I feel would not happen today.