Australia has committed to an unconditional cut to emissions of 5 per cent from 2000 levels by 2020, but would increase the target to 15 to 25 per cent depending on international action on climate change. In a draft report last month, the independent Climate Change Authority found the conditions to lift the target have been met and a 15 or 25 per cent cut should be considered.

If the carbon price is not repealed by May, the government will need to nominate a cap on emissions for companies covered by scheme, which would come into effect from July 2015 when the tax shifts to an emissions trading scheme.

The cap limits the amount of emissions that can collectively come from covered companies - decreasing each year - and requires them to buy offsets if they want to emit more.

But if the proposed caps are rejected by Parliament, then under the carbon laws, default emissions limits for each year to 2020 come into effect. In 2015-16 they would cut emissions 38 million tonnes, and then 12 million tonnes each year until 2020, based on the institute's analysis.

The institute - which supports carbon pricing - says on the basis of the most recent reported data, the default caps would put the country on track for a cut of 11 to 19 per cent in 2020, depending on emissions from industries not covered by the carbon price, and most likely a 15 per cent reduction.