your visa agent was lying about it while probably trying to sell you a different type of visa. Lots of folks on volunteer visas with no problems, just note it has to be with a legitimate organization and you do actually have to put in some volunteer work a few times a month. The scam ones are the ones that claim you don't have to do anything, same with ED visas that promise you don't have to study.
Two different things here. Your IDP is valid for 12 months from issue but you can only drive on it in Thailand for up to 90 days from entry before being required to get a local license.
putting aside that Australia already prohibits Australians traveling abroad without a valid reason, one that requires approval and has done so since about April last year, U.K. is implementing something similar.
Quote from the article below "Lockdown rules mean people must only travel abroad for essential reasons" for traveling outside the U.K.