On arrival into Sydney you should be able to get and load your Opal card at WH Smith in T1. Depending on how much luggage you have (I have no problems with a 69cm Cosmolite), you can then start your visit to Sydney with a small saving by eliminating the
airport station access fee by catching the route 400 bus from the public bus stop outside the terminal to Mascot railway station (alight at Coward St at Laycock St, Mascot Stop 2020113 - fare $2.10) and catching a train into St James from there.
Similarly at Melbourne, if you can get a Myki at International arrivals, the 901 bus from the bus interchange near T4 to Broadmeadows and then a train into the city is cheaper than the Skybus.