Feb 8 1999 Pleistocene Extinction of Genyornis newtoni: Human Impact on Australian Megafauna (Gifford H. Miller, John W. Magee, Beverly J. Johnson, Marilyn L. Fogel, Nigel A. Spooner, Malcolm T. McCulloch, Linda K. Ayliffe, Science, Volume 283, Number 5399 Issue of 8 Jan 1999, pp. 205 - 208 ).
Abstract: More than 85 percent of Australian terrestrial genera with a body mass exceeding 44 kilograms became extinct in the Late Pleistocene. Although most were marsupials, the list includes the large, flightless mihirung Genyornis newtoni. More than 700 dates on Genyornis eggshells from three different climate regions document the continuous presence of Genyornis from more than 100,000 years ago until their sudden disappearance 50,000 years ago, about the same time that humans arrived in Australia. Simultaneous extinction of Genyornis at all sites during an interval of modest climate change implies that human impact, not climate, was responsible.
They were tasty? Or maybe it was easier to kill one big critter to feed the band instead of several little critters.