I've used Transfercar in Aus and NZ, and it has been seamless for me. I'm not affiliated, just a fan of the model.
Just had an email from them that they have a bunch of amazing relocation prices.
The idea is - rental car places often have cars building up at one center (eg say for example, most renters drive SYD to MEL, then fly back, you'd get a build up in Melbourne). They need some way of balancing, so often offer free cars, or cheap cars for a few days. Sometimes fuel is included, or ferry costs, depending on the route. Obviously check the dates, terms and conditions work for you.
Anyway, this one, they have examples like:
and many more. They also have campervans on some routes.