I hadn't actually bothered to listen to the Kookaburra song until now. Yes, the second phrase of the flute part sounds a bit like the opening line of the song, but it's certainly not identical. There are really no more similarities between these two tunes than there are between loads of familiar melodies.
Does it justify a £33m court case? Hell no!
To be honest, I always assumed the flute part was meant to sound like an old Australian campfire song, in a generic, affectionate sort of way.