APT (or Australian Pacific Tours) has been around for many years and is highly regarded in the local travel industry. They, along with AAT Kings, are probably regarded as the 2 elite touring companies in the country. Reputable, reliable, dependable for the most part.
Late March is ideal for Tassie - weather should be ideal as it will be early Autumn/Fall and school vacation periods are mid April this year.

Tassie is very compact though and easy to get around. If you feel adventurous enough, you will easily fill a week seeing almost all there is to do no matter your personal tastes are, by hiring a car and doing your own thing.