The message is as dismal. In summary, it is that the Treasury reluctantly concedes that something needs to be done, but not by them, and that it greatly pains them that they might have to change the tax system as a result of the demise of the carbon taxes that have been such easy revenue raisers for them. Climate change is, very obviously, a great inconvenience in that respect.
If revenues from carbon taxes are falling that means that carbon emissions which pay carbon taxes are falling.
That is, the carbon tax is doing its job of reducing carbon emissions. Or even, solving climate change is a great inconvenience.