You take ferries/ships as needed on such trips, usually with the intent to use them for as little of the trip as reasonable.

Were there international car ferries to/from Australia? I’m not aware of any currently.

As an addition to the other responses about the historical side you can currently still ship your car in/out of Australia today you'll just have to do it as cargo instead of a single passenger ferry which also carries your car too. I know a guy that did that with his car to Europe for ~8 months, just make sure you plan well in advanced or you'll be waiting 2-3 months on that leg :).

Yes, passenger ships and ferries were more common than today. When my mum went to Australia for school in the 1950s she went by ship.

Even in 60s when big himalayan expeditions were done, people travelled by big boats starting usually in India, for a month or two. Then got half of their equipment lost (so this part didn't change that much).

This sounds like a "back in my day I had to take a ship to school" bit

Ships were obviously much more common given there’s basically one trans-Atlantic ocean liner these days.

Yeah a lot of it was killed by cheap flights.

And even before flights got generally cheaper, it's hard for most people to justify taking almost a week longer, between a very limited number of destinations, on a pretty limited schedule.

There used to be far more (sometimes ocean-going) RORO ferries than there are now, Herald of Free Enterprise and Estonia really reduced the market to areas where the ferries are pretty much essential.