A state-controled electricity company has backed down from a plan to end New Zealand's coal-fired generation in 2018, prompting renewed condemnation of the government's lack of action to curb greenhouse gas emissions.

In August last year, Genesis Energy said it would permanently decommission its last two coal-burning generators at the upper North Island's Huntly Power Station by December 2018, but on Thursday the company said it now planned to phase out the fossil fuel-burning units by 2022.

Energy and Resources Minister Simon Bridges said the change was "a short-term security of supply solution" to allow the electricity industry to transition to other methods of generation before the coal units were turned off.

"The announcement today is a pragmatic response that will provide breathing space, while ensuring New Zealand continues down the path of greater renewable generation," Bridges said in a statement.

"Significant market investment in renewable energy has increasingly reduced the need for coal as a backstop in dry years when our hydro-lake levels are low. Huntly's coal-fired units accounted for just 2.6 percent of generation in 2015, compared to around 12 percent 10 years earlier."

The government remained committed to its goal of 90 percent renewable electricity generation by 2025, he said.

In 2015, renewable generation reached 81 percent, the highest level in two decades and an increase of 16 percent since 2008.

"In climate change terms, electricity generation makes up just 6 percent of New Zealand's total carbon emissions," said Bridges.

Critics said the continued reliance on fossil fuels was the result of the government propping up New Zealand's sole aluminum smelter at Tiwai Point, in the far south of the South Island, which global mining giant Rio Tinto has threatened to close and which consumes 14 percent of the country's total generated electricity.

"The energy sector has other options, with almost 4,000 megawatts of consented renewable generation ready and waiting to be built," Green Party energy spokesperson Gareth Hughes said in a statement.

International environment campaign group Greenpeace accused New Zealand's energy companies of cartel-like behavior in plotting to keep coal.

"Rather than invest money in new power sources like wind farms, which are cleaner, cheaper, and create far more jobs than coal ever could, it's easier for our power companies to stick to the status quo," Greenpeace New Zealand climate campaigner Jeff Harrison said in a statement.

Coal generated less than 4 percent of New Zealand's power yet, but accounted for 28 percent of its electricity pollution.

"Just four months ago the government went and made a whole bunch of promises in front of the world at the Paris Climate Conference. And now New Zealand is investing further in coal," said Harrison.