Flying into Cairns from Darwin, driving straight to Port Douglas and will then drop the car off in Cairns 4 days later. Have been looking at Cairns airport website, booking.com, momondo.com and holidayauto. 2 people with 2 suitcases.
A number of these sites have East Coast and Bargain Car Rentals (funny enough not the airport) as well as the usual (Sixt, Hertz, Avis, Enterprise, Europcar) but there is quite a large price difference between these 2 groups. Are there any car rentals that should be avoided in Australia or more specifically Cairns?

We used Hertz in Cairns a few years ago. Everything went smoothly, we've used them in other countries too, never a problem.