It's possible. I play the Renaissance lute, and I know of at least two pieces from the sixteenth century that open exactly like later works. One is "The Earl of Derby's Galliard," whose opening is exactly the same as "When the Saints Go Marching In." The other is a Vihuela piece titled "Pavana de Alexandra," whose opening is nearly a dead ringer for "Deck the Halls with Boughs of Holly."
Imagine the riff in question being the tune of Yankee Doodle - a traditional American tune that any American is likely to instantly recognise. Specifically, the bit around "Stuck a feather in his hat."
Now, imagine if the band in question had released a video clip that not only had one of the musicians playing that riff, but at exactly the same time, he was putting a feather in his hat.
Would you find it hard to believe that was just coincidence?
That is the situation here - Kookaburra isn't as old, but it's been a standard song virtually all Australian children have learned early in their school days since the 1940s. It's been a campfire song since that time as well. Virtually any Australian will instantly recognise it.
And in the Men at Work videoclip, the person playing it most noticeably is a flautist sitting in a gum tree - and the line is "Kookaburra sits in the old gum tree."
Maybe when they wrote the song, it was unconscious - but by the time they made the film clip they knew what they were doing.
The aural comparison is here at youtube.
The videoclip - the guy in the gum tree is at about the 50 second mark.