Decide on the perfect time for this trip!
Dec-Feb: The summer month would be best to go for long days, settled and warm weather and festivals such as Tasmania’s taste of summer in Hobart. Best time to go for hiking and swimming. You can still have 4 seasons in a day and the weather can be very unpredictable at any time of the year. We went at the end of March-beginning of April and felt like the season had just ended. We had a majority of cold, cloudy and some rainy days, even some snow overnight at Cradle Mountain. I think the beginning of March might still work.
If you have the time and you want to do plenty of day hikes, you would not be bored with 2 weeks. We spend just over two weeks in Tasmania and we felt this was perfect for us. We saw everything we wanted to see without being too rushed. If you're planning one of the multi-day hikes, you'll need more time of course.
You can definitely see the highlight of Tasmania if you have one week also. For shorter durations, flying into Hobart and out of Launceston or vice versa is a good way to save time if your car hire will allow dropping off your vehicle at a different location.
Go check out my guide for the best free things to do as well as itineraries and travel tips to make your trip unforgettable.