Pollution isn't the main cause in your first example, massive upriver water extraction for irrigation is.
The Australian blooms were due to a state government selling a huge cotton farm (Cubbie Station) with a ridiculous amount of water storage licenses and the Federal government approved the sale to a foreign purchaser. That might be good and well in times of flood but when we have droughts the water that is stored does not flow through to two other states to get to Bass Strait.
And please don't swallow all the crap that it was the current conservative government that did this. It was a green/labor coalition federally that approved the foreign purchase and a labor state government locally that bought the station, placing an ex labor state treasurer in charge prior to the sale, along with all of the massive water allocations just before the Murray Darling Basin plan was introduced by the current conservative federal government in 2014.