Northern Territory: Mass Death of Brumbies.

(source)

In the same note: informal reports of mass death of camels (!) near Docker River. Both brumbies (feral horses) and camels are introduced species.

UPDATE:
24-01-2019. Rangers culled another 55 struggling horses.

Tim Clancy, from the NT Department of Environment and Natural Resources, said that according to the latest data available, there could be up to a quarter of a million brumbies in Central Australia. Although charismatic among the public, brumbies cause significant environmental damage, particularly during droughts, when they concentrate around waterholes, contributing thereby to erosion and damages to the vegetation.

UPDATE 2:
25-01-2019. Western Australia:

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Another mass fish kill in northern New South Wales: Lake Inverell, on the Macintyre River. The individuals killed at Menindee seemed to have been mostly adults. This time they appear to be mostly juveniles:

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Water bird numbers reveal long-term decline of Menindee Lakes' health
By Peter Hannam. 22 January 2019 — 12:50am.

Related: from the "Eastern Australia Waterbird Survey 2018 Report" (UNSW, Centre for Ecosystem Science).

(source)