Be careful when traveling to Tasmania during any kind of holiday. I made the mistake of arriving during Easter when the buses seemed to shut down.
Getting to Port Arthur from Hobart was impossible by public transport unless you want to get stuck there and stay in prohibitively expensive accommodation. I hear the public transport is slightly better outside of the holidays.
Considering that Port Arthur is historically one of Australias most important places, and that the site is open to the public over the holidays, I was shocked that there was not suitable transport to it during one of the busiest holidays of the year.
Tours from Hobart where a problem as well. Check out different providers because the few I looked at would only give you a few hours to cram in as much as possible before returning and were very expensive. Considering the visitors center at Port Arthur say you should spend a whole day there to see it properly I found it disgraceful that tour providers charged so much to only allow you a couple of hours.
This may have been due to the holidays, but in my opinion holidays are the busiest time for this kind of place, and tours/transport should be better organized.
Port Arthur is an incredible place, with a lot of history and a lot to see. If you visit Tasmania then a trip to Port Arthur is essential, an absolute must see, but if you go during a holiday you need drive yourself or go with friends that do!