Aaah, a southern hemisphere problem. No matter what I say then, the opposite will no doubt be true!
My guess is there is likely to be some setting within your Yahoo mail account properties that relates to character/font encoding. This has been changed or automatically defaulted to some undesired setting. Outlook being a thick/full fat email client is probably able to decode the characters, i.e. the "problem" remains just Outlook deals with it where as the browser and the iPad can't. Have a look at the account options via a browser.