Wikipedia lists the "mighty Murray" as the longest river in Australia and, indeed, it is the river that runs through three different states (New South Wales, Victoria,and South Australia - the Murray River in Western Australia is a different body of water.)
What's more it then lists the Murrumbidgee,which is only in NSW and the ACT. The three rivers are placed here in reverse order. The Darling River has shrunk from a worldbeating length to third place in Australia. I don't doubt the rest of the top 30 river systems so I think it must depend on which part you count as the Murray and which the Darling.
Otherwise what happened? Were there boggy bits they couldn't get to with the tape measure leaving them having to guess? Add in some hostile waterfowl and bite-y insects and it becomes apparent why the Wikipedia waders had a different reading to the midstream monsters, and the role of remote readers barely skims the surface.