Its cheaper by bus the whole way to Bondi Junction, as its all one journey.
The way Opal works, is the first sector charges the bus fare, under 3 kms, (dom to Mascot for you) and then the train fare, will be added on, but then, you get a credit for mode transfer, of a discount of $2, ... for the bus and train fare(ing) system.
Where as by bus to Bondi Junction, its all 8 km bus fare, (straight).
And then the bus to Circular Quay, or as close to it, (haven't gotten the bus there since the light rail came in), so not sure now how far the bus will get to Circular Quay itself.
Bit troublesome and a long ride, but, I always get out to Bondi Junction, though must admit, the journey is very long, via East Gardens and also very windy and can give dizzy spells, but still, I avoid Redfern, (grin), don't ask.
Used to go out to Burwood too, then get the train, if going to Central.
But this was all pre the split of the 350, when that bus used to go all the way from Burwood to Bondi Junction, now I think another bus does the trip from int to Burwood, and the 350 does int to dom to Bondi Junction.
Sorry for the waffly.
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Bus 3 km fare, dom - Mascot - $3.20/$2.24 (peak and off peak)
Bus 3 km - 8 km, dom - Bondi Junction - $3.93/$2.75
Train under 10 km fare, Mascot train station to Central/Circular Quay - $3.79/$2.65
If you add the bus 3 km fare and the train, $6.99 - $2 transfer discount = $4.99 or off peak $4.89 - $2 transfer discount = $2.89
Bus and train $4.99 and $2.89
All bus $3.93 and $2.75