While they attempt to justify this action by environmental grounds, consider the following factors:

They still collect open season permit fees up to the declaration that no season will be declared. They do not return the open permit fees, saying they are necessary for "conservation" - read general revenue;
Due to drought many ducks will die, many more than in a "good" year - so harvesting some only reduces the total population which is in excess of the food supply anyway;
Only a very small percentage of the duck population is harvested anyway;
Populations of ducks also exist away from the major swamps and rivers, on farm dams, smaller rivers and creeks. Again these ducks have limited food supply so ducks hunted does not reduce the end of year population;
There is NO evidence that if a duck season was declared that the population of ducks at the beginning of next season will be any smaller.
Ducks in Australia often do not breed in drought years, but recover numbers very quickly in the next year or two, again meaning very limited harvesting has little affect on the population

What has had an affect over the years is habitat loss. Much of the natural habitat that exists today is a result of duck hunters over the last 100 years, their permit fees, their personal physical exertion. But the "Game Reserves" are now called "Biospheres" and similar BS Guff.

This sort of decision is more based on a dislike of duck hunting and a convenient excuse to stop the duck season for another year IMO.