score:4
ex-Wolli Creek local here. I also used to work for an airline based out of the domestic terminal, and briefly as a contractor on the Opal team at Transport for NSW :D
Getting from International terminal to town
Walk to Wolli Creek (20 min, all paved, not hard at all to find). Catch train (every 3-5 min from platform 1 or 3) to CBD. Cost: $2.52 (off-peak).
Getting from Domestic terminal to town
Walk to Mascot station (20 min, first 5 is a little confusing). Catch train (every 10-12 min) to cbd. Cost: $2.52 off peak.
Getting from town to either airport terminal
With an unregistered Opal card with more than $4ish on it, it's possible to travel to the airport, and tap off, going into negative, at which you can dispose of the card. If you don't have one, they're purchaseable from any corner store for free, min top up $10 (still cheaper than a full train ticket).
They have now installed gates at the terminal, but as of April 2019 it was still certainly possible to do this travel hack.
Upvote:1
You can use the trip planner here to estimate the cost of your trip: https://transportnsw.info/trip#/. Make sure to select "options" and untick the train if you prefer other transport options.
If I search From: International Airport and To: World Square (nearest landmark to Pitt St/Goulburn St), the cost is 3.73 AUD and takes about 1h10min. It involves a combination of the 400/420 bus from the airport to Mascot, and then transferring to the M20 in Mascot.
I don't know if Wolli Creek station is walkable from the airport, but it would probably take half an hour. The cost of a train from Wolli Creek to Central is 2.52 AUD.
Note the costs in this post are all off-peak, it would be more expensive at peak hour.