Entire Australian Coin Collection 1911- 1942

Who would have such a collection?

I suspect your problem is in that phrase "mint condition". If they truly were in mint condition ("uncirculated", to use the numismatic terminology), each and every coin from the 1910s and 1920s would be worth tens of thousands of dollars. This is aside from the actual rarities that fall within that series, such as the 1930 penny, the 1923 halfpenny, and pretty much every florin before 1920, in which case you'd want to add an extra zero to the end of that price.

If, on the other hand, the collection was comprised of coins in "typical circulated" condition, and it was missing the gold coins, the 1930 penny and the 1924 halfpenny, and the early florins were all so heavily worn that they are barely identifiable slugs, then the collection would be worth a few hundred dollars, maybe a thousand or two if some of the coins were nicer. At least, that's what a dealer would offer, and only a dealer would be interested in buying a "full set", since most collectors would already have a partially-completed set of their own and would be reluctant to pay full price for duplicates.

It's difficult to find a retail price for such a set, because 1911 and 1942 are unusual cut-off dates, with the 1910 coins excluded and 1942 being partway through the reign of George VI and part-way through World War II. Most people selling sets are selling either sell a "full predecimal set" (1910-1964), a "George V set" (1911-1936), a "George VI set" (1937-1952), or a "World War II set" (1939-45).

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