I have just come back from visiting the lovely Cutty Sark in Greenwich - and I made a note of a display that I wanted to know more about. It says:
"Most of the seamen who sailed on the Cutty Sark did so just once. They were paid off when they returned to London and had to look for another ship. But many took the opportunity for a free passage to Australia. In 1876 for example, 12 of the 15 able seamen deserted. they had to be repalced by expensive Australian seamen."
Why did they only sail on such a lucrative and prestigious boat once?
Why were Australian sailor so expensive?
In answer to the part of the question concerning Australia. The 1870s were the middle of the various big Australian gold rushes. Many ships, on arrival at Australian ports, found large portions of their crews deserting and heading for the gold fields leaving them short handed. To sign on new crew in Australia ships masters had to compete with other vessels for a limited supply of sailors. This made replacing crew in Australian ports expensive. Note that the sailors were not particularly of Australian origin.