She also cast doubt on Coalition claims that there is $50 billion in unallocated revenue from the sale of carbon credits, saying she would like to know how the Opposition reached that figure.

Senator Wong was speaking after the Government reintroduced its legislation for an emissions trading scheme into the House of Representatives yesterday.

It needed to introduce the bills this week to leave open the possibility of a trigger for a double dissolution before global climate change talks in December.

Speaking in Parliament on the introduction of the bills, the assistant Climate Change Minister, Greg Combet, appeared to cast doubt on Coalition amendments exempting coalmining and agriculture from the scheme.

''Any proposal to exclude or carve out sectors from the CPRS must be examined very carefully,'' he said.

''Any benefits have to be weighed up against the increased burden on other industry sectors, the loss of opportunities for low-cost emissions reductions, and the possible loss of permit revenue to assist households.''

Senator Wong would not reveal what the Government had forecast as future earnings from a scheme, saying governments did not normally reveal forward estimates beyond a certain year.

''There is no bottomless pit of money when we look at this legislation,'' she said. ''They've put a figure on the table, and I'm certainly interested in understanding how they've arrived at that.''

The Opposition says amendments it is negotiating - which include giving much more compensation to heavy industry and coal-fired electricity plants - would not add to the federal budget.

The Coalition says it will use money from the sale of carbon permits after 2015 to pay for the compensation, which has been estimated to cost $8.9 billion in the first five years of the scheme.

At a Senate estimates hearing yesterday Treasury officials said households faced a $78 price rise in electricity in the first year of the scheme, followed by $146 in the second. The Government proposes to compensate households for price rises in the second year.