Don’t forget some warm clothing as well- Portland enjoys the coolest summer temperatures in mainland Oz outside of the Alps (and roughly comparable to northern Tassie on average). In December the ocean will still be chilly. Of course when the wind is northerly it will be warm-hot as with most places in VIC.
I’ve never done the ‘Wally Rail’ but it’s on the to do list when I’m next down that way. It’s actually doable as a day return trip from Melbourne, if you are a fan of long train trips.