The coalition says Climate Change Minister Greg Combet needs to stop name-calling and start redoing the government's carbon price numbers.

Mr Combet has admitted the federal government may have to revise down its revenue forecasts if the global price of carbon doesn't pick up in the coming years.

When Labor planned its carbon tax, it had anticipated the price would rise when it switches from a fixed to a floating price in mid-2015.

The fixed price is $23 a tonne and was expected to rise to $24.10 by July 2015.

But Climate Change Authority (CCA) modelling reportedly shows the global carbon price is expected to fall, risking a $4 billion revenue hole in the federal budget.

Opposition environment spokesman Greg Hunt said the carbon tax regime was "in chaos" and it was time for Mr Combet to concede it would either deliver skyrocketing electricity prices or a slap to revenue.

"Mr Combet must stop the name calling and start redoing the figures and being honest with Australians," he told reporters in Canberra on Thursday.

"We have warned for a long time that the government's figures were unreliable."

The climate change minister on Wednesday described Opposition Leader Tony Abbott as Australia's top "bulls..t artist" because of his claims the carbon tax would be a wrecking ball through the economy.

CCA chairman Bernie Fraser has told The Australian Financial Review that Treasury's predictions the carbon price would recover in value were unrealistic and might need readjusting.

The CCA factored into its forecasts a view that the carbon price could slump to $10.72 by 2015/16.

Government updates to the forecasts are expected to be included in this year's budget.