The New Zealand government will be challenged to quicken its step to cut greenhouse gas emissions when it opens its post-2020 targets to public submissions this month.

The national target to reduce harmful emissions after 2020 would be taken to international talks on a new climate change agreement in Paris in December, Climate Change Issues Minister Tim Groser said on Thursday.

"New Zealand wants to set a target which is environmentally credible and reflects our particular circumstances. But we also need to consider the possible impacts and costs to our economy," Groser said in a statement.

"Increasing our commitment after 2020 will be a big challenge, as nearly half of New Zealand's emissions come from agriculture and 80 percent of our electricity already comes from renewable sources. The easy gains have already been made. But we are expected to make a fair contribution to combating this global problem."

Groser said the government had released a discussion document and submissions would close on June 3.

The government has three targets for reducing greenhouse gas emissions: an unconditional target of 5 percent below 1990 emissions levels by 2020; a conditional target range of 10 percent to 20 percent below 1990 levels by 2020, if there is a comprehensive global agreement; and a long-term target of 50 percent below 1990 greenhouse gas emissions levels by 2050.

It aims to meet these targets through a mix of domestic emission reductions, the storage of carbon in forests and the purchase of emission reduction units from other countries.

However, it has come under fire for weakening the country's emissions trading scheme and for allowing deforestation to outstrip forestation in the near future.

The opposition Green Party said government policies were driving an increase in harmful emissions and called on it to at least match the European Union's pledge to reduce emissions by 40 percent by 2030.

"The government's consultation document is seriously flawed. It treats action on climate change as a cost, whereas, in fact, failure to take action is actually the cost," Green Party co- leader Russel Norman said in a statement.

"The New Zealand Treasury found that if New Zealand continues on its current trajectory of increasing emissions, the cost to taxpayers of even a modest 5-percent reduction target will be up to 52 billion NZ dollars (39.01 billion U.S. dollars). Reducing our emissions will lower this cost," he said.

"The reality is that the government's farming, roading, forestry and energy policies are driving dramatic increases in our greenhouse gas emissions, projected to grow by a massive 50 percent in the next decade."