It's not a very close resemlance, but it's definitely there, and as Caellach said - why shouldn't it be?
It's been considered very irritating because the original woman who wrote the song never ever recieved any royalties for its use - because she didn't charge them. Only after her death (and long after "Down Under" was released) did Larrikan music get hold of the copyright. Even then it wasn't until the late 00s that it was pointed out on a music trivia show (curse you, Spicks'n'Specks!) did anyone even NOTICE the similarity. It's one of those things that, once it's pointed out you see it, but not until then. THEN they've suddenly gone "Oh noes! They've ripped us off!"
To put this in context, during the America's cup challenge in 1983 (84?) this song was basically the national anthem. If you went a day without hearing it it would be a miracle. No-one even noticed this 'rip off' because, well, although awesome, it's not the point of the song.