There's no doubt the Greens, who'd like petrol covered, would have to concede this point. Bob Brown knew this and has been signalling as much for months.

We haven't got the full picture on fuel yet. The government is releasing only half droplets of information about its carbon plan. So we know that about 12 million motorists, including tradespeople and families, will be protected. But some - as yet unspecified - industries will be hit. The government has given out the good news while keeping back the painful aspect.

The motorists' exclusion weakens Opposition Leader Tony Abbott's scare campaign. On the other hand, the Greens have won a concession that gives some ammunition to Abbott. The deal includes a Productivity Commission inquiry, reporting after three years, to examine fuel excise (in the Greens' words) ''with a view to shifting the tax base onto the carbon and energy content of fuel''.

Greens leader Bob Brown yesterday made it clear that ''down the line'' he thought it inevitable that ''all fossil fuels will . . . pay the full cost of the creation of climate change''. That's what the Greens will be advocating.

But Prime Minister Julia Gillard said yesterday: ''The design of this scheme is that petrol pricing will be out now and out for the future.''

Talk by the Greens or the government about the long-term future - or for that matter by the Coalition - has an air of unreality. But the immediate political point is that the Greens' comments and aspirations about eventual inclusion of petrol undermine the reassurance Gillard wants to give motorists that they'll be protected forever.

The Greens have to talk up the concessions they have got - which also will include some big money for renewables - because their compromises on petrol and other things will disappoint the purists among their base. Labor and the Greens have common interests in the carbon price deal but separate ones too, which will be at odds.

Michelle Grattan is Age political editor.