The Murray River flows to the sea at the port city of Adelaide. I am not aware of the salinity in the basin, but irrigation causes salt buildup when the water used to irrigate evaporates, leaving the salt behind. This eventually destroys agricultural land, with the only way to get it back being to allow the rivers to flood, thereby dissolving the salt. I don't know if the situation is worsening, but given that the human population of the Earth continues to rise, it probably is.

A much better example of this process is in the drainage of the river Euphrates, which flows through modern day Iraq. The floods of the Euphrates and the fertile valley were once the basis of one of the world's first human civilizations. Then it was dammed, climate change caused it to try further, and poor land use resulted in the topsoil eroding away. As a consequence, Iraq is a very barren desert today.

First, I have to apologize, because I don't know very much about the Murray Darling Basin. I can tell you a few things about salinity though.

Water dissolves small amounts of salt from the rocks, sediment, and soil that it flows through. "Salinity" is just how much dissolved salt is in the water relative to how much water there is. When some of the water evaporates, the dissolved salts are left behind and the remaining water becomes more "saline" or "salty," because it has the same amount of salt but less water. If most or all of the water evaporates then salt crystals will start to form.

In dry places like the western USA or Australia, water evaporates faster and leaves behind more salt in the soil, lakes, and rivers. This makes it very hard to irrigate crops, because most plants can't live with very much salt in their soil.

I appologize if you knew all of this already, but I hope it helped!

P.S. The Australian government has a great website about the salinity problem in the Murray-Darling Basin, if you have not seen it yet!