(earlier post seemed to disappear after hitting submit - so here goes)
By considering only CO2 emissions, the whole benefit of the "carbon tax" is not being considered. There are other emissions that contribute to greenhouse warming, and also other emissions that are outright toxic- all this is captured under the current National Greenhouse and Energy Reporting scheme (and if anyone is interested, this scheme is much more akin to an accounting standard than as a pure environmental scheme).
As for the matter of international competitiveness, consider that ‘emissions-intensive trade-exposed’ industries are already awarded free carbon units because of exactly that, to ensure they remain internationally competitive while other nations are still developing their schemes. The Wikipedia link i gave earlier lists who received those free units.
There is another question that can also be asked? who has actually been effected by the carbon tax to date, that they've changed their behaviour? To most of us as individuals, the impact is going to be through the price we pay for electricity, and it should be noted that the steep increases in electricity prices of late have very little to do with carbon tax.
The Carbon Pollution Reduction Scheme was never intended to be an overnight change, but to occur over a longer time period, to allow industries to adapt and find solutions.