Is Australia the largest island? I had participated in a quiz recently and one of the questions was 'which is the largest island in the world?'. I wrote Australia as the answer remembering that it was the 5th largest country in the world and none of the top 4 were islands.

However (and rather dreadfully) the answer given was Greenland.I took this up with the committee providing the solutions, but they simply said that Australia isn't an island and is a continental mass?

I googled it and found the same argument online which seemed to confirm that Greenland was the largest island.

But Australia is a country and the collection of islands which is a continent (more accurately described as Oceania ) is sometimes called Australia. Neither of these names would still change the fact that Australia (the country) being an island is the largest island in the world! Can someone please clarify this for me?
Australia does get widely referred to as an island as well as as a continent - an island-continent sometimes. My understanding is that "continent" in terms of geology refers to the largest land mass of a continental plate and is more a matter of convention - a general agreement - than precise definition. Such definitions have been subject to revision. 

This definition would have come after continental plates and plate tectonics were confirmed and widely accepted; proposed first by Alfred Wegener in 1912, acceptance did not come until the 1950's and 60's. Naming Australia an island predated this definition and still persists in popular usage but using the current definition it is a continent. 

Greenland, being a part of the North American Plate but not it's largest landmass, gets defined as an island and is now considered the world's largest island.