Despite reservations from senior colleagues, the Opposition Leader said yesterday he preferred to start a scheme in 2012 but with conditions.

"The commencement date should occur in the context of having firm commitments from the rest of the world in terms of what they are going to do," he said.

"If Australia acts unilaterally without commitments from the major emitters, the United States, India and China in particular, we will do enormous damage to our economy without any appreciable environmental gain at all."

Labor's "ambition" is to begin a domestic emission trading scheme in 2010 in the belief that Australia needs to join other nations that have already acted, in order to persuade the big polluters.

The Coalition will oppose this because it believes 2010 is too hasty.

It promised before the election to introduce a domestic scheme by 2012, without the conditions Dr Nelson is imposing.

Dr Nelson first outlined the conditions on Monday. His office claimed he was misreported and Dr Nelson reassured senior colleagues on Tuesday and again yesterday during phone calls that he had not shifted policy.

The colleagues included the deputy leader, Julie Bishop, the shadow treasurer, Malcolm Turnbull, and the environment spokesman, Greg Hunt, and all subsequently stated publicly the Coalition was committed to a scheme starting 2012.

Several backbenchers also complained and urged Dr Nelson to show some leadership.

Mr Turnbull said yesterday that the scheme should start in 2012 but could be implemented gradually until the major polluters were on board.

"Only a global solution will affect global warming. It's not an excuse for us doing nothing but we've got to be smart about it," he said.

Mr Hunt said Dr Nelson supported "a hard start-up date of 2012" and it was Labor that was confused because it had released scant details of its scheme so far.