The limitations of regulation
There are so many ways in which people can reduce greenhouse gas emissions. They can change spending habits and buy different types of products. They can buy the same type of product, but choose ones that have a lower carbon cost. They can switch off lights when they aren't using them. Industry can use electricity, heat etc more efficiently. They can completely change how things are made. They can fix leaks in the compressed air system. They can decentralise to reduce transport requirements. Buildings can be made far more energy efficient at little or no extra cost. Electricity production can change too. Carbon could be sequestered. Waste heat can be recaptured. Transmission efficiency can be improved. They can switch to alternative sources.
All these options. How could the government possibly know which is cheapest, and how could they possible mandate change effectively? They can't. It shouldn't be up to the government to figure that out. It should be up to the government to ensure people consider the full cost of their decisions, and they should let all the economic forces drive the most efficient changes.
You can bet that the cheapest changes wouldn't be more wind turbines and it won't be people switching their cars to gas, both of which are being heavily subsidised in Australia. There are far cheaper ways to reduce emissions.