Justice Peter Jacobson said the famous flute riff from the pop hit was unmistakably the same as the children's tune Kookaburra Sits in the Old Gum Tree.
"I have come to the view that the flute riff in Down Under in the 1979 recording and 1981 recording infringes on the copyright of Kookaburra because it replicates in material form a substantial part of Ms Sinclair's 1935 work," Justice Jacobson said.
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Kookaburra Sits in the Old Gum Tree was penned more than 75 years ago by Melbourne teacher Marion Sinclair for a Girl Guides competition.
WATCH the video clip for Down Under and decide for yourself
The judge said publishing firm Larrikin Music, which was suing EMI and Down Under songwriters Colin Hay and Ron Strykert, had succeeded in their bid to recover unpaid royalties and losses by proving the similarities between the songs.
But he said a Qantas advertisement, which also used a small similar section of the riff, was not in breach of copyright laws.
Compensation yet to be decided
The parties will meet again on February 25 to discuss the findings and begin discussions about costs.
Larrikin Music's lawyer Adam Simpson says EMI and Down Under may be forced to hand over as much as 60 per cent of their earnings from the international hit record.
"It's a big win for the underdog," Mr Simpson told reporters outside the court.
When asked how much Larrikin would be looking for, he replied: "Obviously, the more the better but it depends - anything from what we've claimed, which is between 40 and 60, and what they've suggested which is considerably less."
