Melbourne to Adelaide via the Princes Highway is 906kms. The travelling time is 11 hours. If you take the Great Ocean Road it is slightly more kilometres (132 - total journey 1038 kms) and significantly more tiring - though certainly well worth it for scenery. We drove from Warrnambool to Geelong in one day and were pretty exhausted. I would strongly recommend a break before reaching Warrnambool from Melbourne - say Apollo Bay. I believe 3 days would be sufficient for the journey.

Kangaroo Island is fun but much bigger than I was expecting. A tourist site warns that a day bus trip on Kangaroo Island will cover 370 km in that day - the seals & penguins are at opposite ends of a quite big island! You might wish to be aware that there are quite a few unsealed roads and you will need to discuss these with your car hire company - car hire in Australia usually says you may not take their car on unsealed roads and if something happens the insurance won't cover you.

I assume you are aware that you can't get from Kangaroo Island to Tasmania using ordinary transport except via Adelaide. If you were planning to take a car to KI then you might go there before Adelaide and take the ferry from Cape Jervis.

Regards Anne Canberra, Australia