Only around 500 of Australia's pollution-emitting companies will pay the carbon tax, despite repeated statements by the Government that the "1,000 biggest polluters" will pay.

The decision to exclude fuel for private motorists and small businesses from the scheme means petrol refineries and distributors are out, while firms emitting synthetic greenhouse gases, such as the refrigeration and air-conditioning industries and those that make fire extinguishers, will also be excluded under a special arrangement.

The 500 largest polluting companies who will pay the tax include coal-fired electricity generators, mining companies, and steel and aluminium manufacturers.

Meanwhile Fairfax newspapers are reporting the starting price for carbon pollution will be $23 a tonne - a compromise between the Government's preferred price of $20 and that of the Greens, who wanted a far higher price to encourage investment in renewable energy.

So far Cabinet has not seen the details of the carbon tax deal which was thrashed out in the Multi-Party Climate Change Committee (MPCCC) by the Government, Greens and independents Tony Windsor and Rob Oakeshott.

Cabinet will meet on Saturday to hear details of the scheme while ALP backbenchers will be briefed on Sunday morning just ahead of the official release.

While the deal excludes individual motorists and tradespeople from paying extra for fuel, the Government has skated around questions about how much heavy transport firms will have to pay.

However, it has blunted Opposition Leader Tony Abbott's warning of a possible six cents a litre impost on the price of petrol for ordinary motorists.

Greens MP Adam Bandt says the exclusion of petrol from the carbon price is "disappointing".

"For us that's disappointing that a carbon-based fuel isn't going to be included but, look, at the end of the day the most important thing is that we reach an agreement that allows us to take the first steps towards rapidly cutting pollution."

While the Government promised a revenue-neutral package, it is understood the compromises reached within the MPCCC mean the scheme will cost $4 billion over the first four years.

AM has been told the shortfall will be made up by savings cuts, such as to diesel fuel rebates.

The Government has promised nine out of 10 households will be compensated for cost-of-living increases. Treasury modelling for the previous CPRS, based on a $20-per-tonne carbon price, calculated it would cost the average family about $400 a year.

Mr Windsor refused to confirm details of the scheme, but urged people to take the time on Sunday to carefully go through the plan and acknowledge the efforts that have gone into addressing climate change.

"The easy thing for all of us, the Governments, the independents and to a certain extent the Greens too, would have been to have left this to another parliament.

"This is a fairly gutsy thing for anybody to be involved in in a hung parliament where you don't have capital to burn."

Mr Abbott described the new figures as "sneaky and tricky".

"Have 500 firms stopped polluting? Is their pollution no longer something to worry about or are we only getting half an announcement on Sunday and going to get the rest of the bad news after the election?" he said.

"The more we hear, the sneakier and the trickier this whole business looks."

And he seized on reports that the Greens will be briefed on the details of the package before Labor's own Caucus.

"What has the Prime Minister got about her own backbenchers that she doesn't trust them with the details but she does trust the Greens with the details?

"The most complex structural change in Australia's history has been cooked up by a gang of six. One Prime Minister, one minister, two Greens and two independents. It's no way to run a country and frankly if we have bad process we're going to have bad outcomes."