March/April is generally not a peak tourist season for Tasmania | Melbourne | Perth, but having said that, it is also a great tine to visit all three destinations; with luck the heat intensity of summer will have mostly passed on the mainland, while Tasmania won't be cool yet.
The only thing to watch out for is Easter, and the ten days of school holidays after it, since this is usually a busy holiday period. Easter 2016 starts on Friday 26 March, and I would be booking vehicles and any accomm now, for this period.
