From an American non-musican perspective:

The bulk of the song "Down Under" stands on its own and is a jolly good song.

The riff most definitely sounds like a play on "Kookaburra" while not being identical to it (We used to sing Kookaburra in elementary school, first verse only, in rounds, and even modern young Americans seem familiar with it). I think it is fair to say the Men at Work song did a musical reference to Kookaburra, identifiable on a subliminal level, if not a quote, and musically it was well worth doing.

If I was to be street judge, I'd say it was no way plagiarism but there is a relationship which profits both songs, and maybe Down Under should throw a little something into the Larrikin kitty, but not millions.