We had used our Canadian CC's at many shops in Cairns and Sydney, no problem.
As for taking train in Sydney, if you don't have opal card, you can use a CC that has Wave technology, and most CC's these days would have that. We took the train 2 nights ago, to the Circular Quay station. At the entry station turnstile we waved a CC, and at the Circular Quay station exit turnstile we waved the same CC. It automatically calculates your charge. So, if you don't have opal card, no problem to just use a CC.
But we'll probably use uber ourselves. This morning I checked the price of uber at a bit after 10am (forgot to check earlier) and it would've been AUD$33 from Marriott hotel (by Circular Quay) to airport. I don't know what the cost would be at say 8am, morning rush hour time. The big difference is the uber car comes right by the OPT. You just come out of the terminal and the road is right there, so you don't need to lug the luggages to the Circular Quay station. This morning while pushing our wheeled luggages, several times a wheel would get stuck in the holes of the brick road.
For my wife and I, taking the train would be AUD$39. Even if uber is a few dollars more, we'd still take uber for the convenience.