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Environment Minister Nick Smith has likened New Zealand's adoption of an emissions trading scheme to war efforts in Gallipoli and Afghanistan.

He told the National Party northern conference in Waitangi at the weekend that it might not make a lot of difference to the overall result, but it was about New Zealand doing its fair share.

Dr Smith was being questioned about the scheme, which is about to be expanded on July 1 to include transport fuels and electricity production.

"The challenge I give back to you is: when our Anzac troops went to Gallipoli, and when we've got our New Zealand troops in Afghanistan, do we really think those New Zealand troops in Afghanistan are going to make a world of difference to the final outcome there?

"No, we don't. But what we do say, as New Zealanders, and what those Anzacs said in the tradition of New Zealand, is we as a country believe in doing our fair share."

A bid by some National Party members to delay the emissions trading scheme was defeated at the conference after Dr Smith spoke.

A resolution was lost on a show of hands that appeared to be about 80 per cent to 20 per cent.

Dr Smith and later Prime Minister John Key said the Government was about to embark on a public relations campaign to defend the ETS.

It will involve a letterbox drop to households as well as at least 30 meetings up and down the country addressed by Dr Smith.

Mr Key repeated his commitment that if the 2011 review of the scheme finds that "the rest of the world isn't doing more, we'll slow it".

An Act Party campaign to delay the scheme found favour with many farmers, including some in the National Party.

National and Act agree on one set of figures, that the estimated levies for fuel and power will cost the average farm $3900 from July, a further $3900 from January 1, 2013 - when the scheme is expanded again - and $2400 from 2015 for methane emissions and fertiliser.

Dr Smith said National had halved the costs to businesses and consumers of Labour's ETS, with an increase of about 3.5c a litre on fuel and 5 per cent on the price of power.

"These cost impacts need to be kept in context. The cost to an average dairy farm of the fuel, power and processing impacts of the ETS is 0.5 per cent of returns."

Dr Smith said proceeding with the ETS was also about "honouring our word to voters, to investors and to the international community".

"It's also important we honour our word to foresters. Both National and Labour Government's exhorted them to plant trees with the promise they would receive the benefit of the carbon credits," he said.