From what I found online, apparently three to six months is the default period, which is supposed to be set to match the purpose of your trip, i.e. to give you enough time to conclude your business or tour. And knowing our bureaucrats, they wouldn't let a dangerous Australian have anything besides "the usual" simply to protect themselves from being told "that's not what we do". The phrase here is "CYA" and no, that's not a nautical club.
Country Reciprocity Schedule
But our State Department indicates they have a one-year reciprocity with Australia on visa entries, which would appear to mean that you CAN be allowed one year on the first entry, if you can explain why you need one year, why you're not a threat, and the man stamping the passport knows how to count that high.
I'd suggest corresponding with our State Department, explaining that this is a recreational vessel and your travel simply is s l o w and to a remote area, and confirm that twelve months is both appropriate "from what I have read on your web site" (i.e. prompt them along) and suitable. If you can get that acknowledged, bring the letter or printouts in with you, and let the ICE guy see it and read it before they stamp you in. Assuming that you actually get acknowledgement that 12 months is acceptable.
That doesn't guarantee they will, but usually if these folks see something in writing "from upstairs" they will go along with it. Once anything has actually been STAMPED and put in writing--these guys are extremely reluctant to override or change it. If they indicate they don't do 12 months, ask politely to see a supervisor, again show him the papers. And of course, let the supervisor know Juneau is landlocked
. Most Americans have no idea how things are in Alaska and the very idea of a state capital being that isolated is unthinkable.
Then when you get to Juneau, you can let them explain why the bronze outside is of three pelicans
in a state where those don't fly, either.