Yes, AUD is generally accepted everywhere in Thailand, with not much of a rate difference.
But in Cambodia and Laos, USD rules, so you better change some USD in Australia first. The exchange rate depends on location and establishment (Bank's rate is worse than Money Changer, with Airport giving the worst rate)
Vietnam uses USD less extensively than the above two, so it is recommended to change some dong for small transactions.
Note: Under no circumstances should you allow a bank or a MC to charge you a commission on cash. This is not how it is done in this part of the world.