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.
Not supporting plagiarism, but American music “borrows,” legitimately, all the time — it’s an artistic device, not a substitute for an original idea.