Johnny, sales tax will vary for each county in each state, and sometimes by the city as well. You need to enquire where you will register the boat--if you get a state registration.
But state registration will not be recognized as proof of anything, once you leave the US. For that you need titling. Since you are an alien, you cannot get USCG documentation, which is the US federal titling. In order to bring the boat into Oz, you may be required to have Australian titling on it--and to pay any tariffs that are in effect at that time.
Local (state) registration and taxation can be required if you are in any one state for as little as 30 days. On the 31st day, registration, insurance, and taxation can kick in, because in most states boats are treated as motor vehicles and motor vehicles are required to be locally registered, often in 30 days, sometimes as generous as 90. In some places you cannot get a marina contract without showing proof of local taxes being paid, the tax men are ahead of the game and the marinas don't want to get stuck in the middle.
Your best best? I'd say find out what Oz wants. Catch-22 applies, because if you place the boat under any non-US flag, you need to remove it from the US, get a cruisoing permit, and then reimport the boat--for a period not to exceed one year before you must exit again.
These kind of regs don't just apply to the US, they apply everywhere. Everyone wants a piece, and don't even think about sailing home to Oz without advance clearance as your arrival date becomes closer.