[smoo]

The expensive part is what I assume to be a one way rental.
They need to relocate it back to SA, so they say.
If you’re doing it online, actually ring up the main rental companies and ask if they need a one way or return rental from Adelaide to Tullamarine. It might not show up on their website when you tick the boxes.
Also ring up the camper van mobs and see if they need one taken to Melbourne.

A corolla through Avis for 24 hours, dropped off at the same location with excess reduction is around $140.