Labor's leadership crisis has exposed concerns about the $23 a tonne starting price for the tax, with Mr Rudd and his supporters indicating fears the price is too great a slug for business.

But Gillard backers are standing firm. Senator Wong said on Sky News yesterday that the carbon compensation, including tax cuts and higher benefits due to come in from April, are fixed.

''You'd have to be very careful that that didn't have a very negative effect on the budget. Because if you move to a floating price, obviously household compensation quite rightly is fixed. There's obviously budgetary risks there,'' she said.

Mr Rudd has said he would like to review the carbon tax after the first six months and indicated he would like to move more quickly from a fixed price to a floating price.

The carbon tax, due to start on July 1, will begin at a fixed $23 a tonne, rising slightly through to 2015, when it shifts to an emissions trading scheme with a price that is set by the market.

Businesses have expressed worries that $23 is too much. The Energy Minister, Martin Ferguson, a Rudd supporter, echoed these concerns yesterday.

''I think there's a lot of concern in industry at the moment about the price we've locked in given where Europe is at the moment in terms of price of carbon - whether we've locked in a price that's to our disadvantage as a nation,'' he told the Ten Network.