Official documents show Climate Change Minister Tim Groser was considering introducing a tougher emissions target than what was eventually settled on.

Photo: AFP

Earlier this year, the government announced it was targeting reducing emissions by 11 percent below 1990 levels by 2030, in preparation for a major United Nations climate change conference in Paris later this year.

The Treasury recently released a swag of documents on the advice it provided the Government on the decision.

An email from a Treasury analyst shows that they believed Mr Groser was going to recommend a 15 percent emissions reduction on 1990 levels, because it would have more credibility in international negotiations.

The documents show the extra cost of a 15 percent, relative to a 10 percent target, would be $2 billion over the course of the 2020s.

The email recommended Mr Groser be questioned over what the additional benefit of a 15 percent reduction was, and whether it was greater than the additional cost of $2 billion.

There was also a suggestion that the minister might recommend New Zealand look at a shorter period of time for its target.

The email said it was understood Mr Groser may suggest the government take its post-2020 target over 2021 to 2025, rather than 2021 to 2030.

It said this would require more effort to reduce emissions over the 2020s, and thereby increased the costs of meeting New Zealand's target.

A range of proposals were discussed but ultimately Mr Groser took a 10 percent target to Cabinet.

Mr Groser said he always looked at the alternatives on policy issues.

"We looked at alternatives above where we came out. We looked at alternatives below where we came out.

"The only thing that matters is where we collectively agreed at a cabinet, and I think we've come up with a very respectable target."

Meanwhile, the Labour Party is urging Mr Groser to support protections for climate legislation from trade deals.

A new report suggests governments need to be protected from provisions in trade agreements that give corporations the right to sue for losses caused by climate change policies.

Such provisions are already in some deals New Zealand is signed up to, and are also included in plans for a Trans-Pacific Partnership.

The paper was prepared ahead of a major meeting in Paris, where it is hoped the first comprehensive global treaty to combat climate change will be agreed.

It recommended a provision be added to the Paris deal to protect governments against the risks of legal action being taken under so-called Investor State Dispute Settlement provisions in various trade deals.

Labour climate change spokesperson Megan Woods said the suggestion made sense.

"It would allow governments around the world to be confident when they are making the policy that they need to make to tackle climate change, that they are going to be able to do that without the fear of drawn out legal action or large fines.

"I think the plan is sensible, and it's something that i certainly urge the minister to have a very careful look at."

Mr Groser's office did not respond to a request for an interview.