It was surprisingly easy to fly with our bikes. We didn’t even have to pay any extra, as with Virgin Australia, any sports items can be checked in as a hold bag.
Maximum weight without being charged extra is 23kg, though both of ours were a few kg over. Most flights with Virgin Australia have a length limit of 1.8m and no other restrictions, so a cardboard bike box is fine. You’re also entitled to two items of cabin baggage (about the size of a Ortlieb rear pannier), technically weighing no more than 7kg altogether, but they never checked ours.
We did spend almost a day cleaning and packing our bikes and bags for the flight, as we had heard that immigration can be pretty strict, and didn’t want to risk our bikes being quarantined, but when we arrived in Australia they just waved us through security.