A Sheila is a pretty young woman with every imaginable good quality that an Aussie guy could possibly desire. It’s a compliment of the highest order. A term of endearment. To others, it’s any female.
Some say it originated from the Irish name, Sheelagh, for which there were many immigrants in the early 1900s into Australia. So the shortened version of Sheila would have been a term of endearment from the hordes of Aussie guys trying to charm them.
Others say it was a generic term for an Irish woman, as “Paddy” is generic for an Irish man. Though not a disparaging term, men did not call a woman “Sheila” to their faces, unless it was her name, which could be a little awkward :)