The Australian spelling dictionary aims to provide the single spelling preferred in Australia, but sometimes, as in this case, it can be quite difficult to identify the single preferred spelling. A search of Google for sites from Australia (sites ending in .au) returns 5.41 million results for free standing and 6.37 million for freestanding. Because of the way search engines work, searching for free-standing with the hyphen, usually doesn't return something we can have confidence in, as a hyphen is often treated as a word delimiter. Hyphens are also regularly used in the website URL (address) and these results will be returned by Google as well.
The Macquarie dictionary has the entry freestanding, but not free-standing. The Oxford Australian dictionary has free-standing, but not freestanding. Microsoft Word is no help at all as it will suggest freestanding and free-standing with the same meaning, and both free and standing are legitimate words in their own right.
It appears this may be a situation where the spelling is evolving. Often when there's two words used as an adjective, the words will be hyphenated. However, as time goes on the hyphenated words drop the hyphen and the result is a single word. With freestanding exceeding free standing (and free-standing), this would indicate the preferred Australian spelling may now be freestanding, with the hyphenated version free-standing a secondary variation. The use of free standing as a two word adjective - when used in the sense of self-supporting, unconnected, independent – which is very common, would also not be the preferred spelling.
Kelvin Eldridge
The preferred Australian English spelling dictionary
www.Australian-Dictionary.com.au