What visa options are available for an Australian passport holder visiting Thailand from Bali for two months?

You got two choices, you slog on over to the thai consulate in Jakarta and BUY a single entry tourist visa from them which will stamp you in for 60 days when you arrive and which you can extend one time for 30 days at the immigration office

OR

You come in on a free 30 day entry stamp (called a visa exempt entry) and take the one 30 day extension you're allowed on that at the immigration office after you're here

Either way will work out,

Definitely CHECK with your air carrier though because some carriers (especially ones here in S/E Asia) require you to have proof of onward travel within the days you'll be stamped in for if you're comin' in free entry (so in your case you'd need to show you're leaving in 30 days no matter what you want to do after you're here)