Duncan Gillies is shocked by the ever-growing mountains of plastic at the Huntly Transfer Station

Kiwis are conscientiously sending their shopping bags off to be recycled or composted, unaware most are being thrown in landfills.

The crisis in composting and plastic recycling capacity has prompted calls for councils and the Government to step up and do their bit.

Conservation Minister Eugenie Sage has promised a decision on banning plastic bags in the coming month. Industry insiders predicted any ban would be phased in over several years.

SUPPLIED Nick Morrison of the Bags Not campaign is lobbying councils to install hot composting bins.

This weekend Dr Trisia Farrelly, co-director of Massey University's Political Ecology Research Centre, called for the government to take the lead and ban harmful plastics. "We have got ourselves into this situation we shouldn't have got into. There needs to be an international legally binding directive. We have reached a plastics crisis," she said.

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Plastics have been piling up around New Zealand ever since China slapped restrictions on imports of waste products, at the start of the year. The sight of massive stacks has caused alarm around New Zealand: a Huntly resident compared the piles at his local waste station to "the slums of Mumbai". On the West Coast, Smart Environmental operations supervisor Allan Corbett told the Westport News that "nobody has the answer" to piles building up at sites like the town's transfer station.

And as 254 Warehouse Group stores switch to compostable bags, it has emerged that there as few as 11 composting plants in New Zealand able to handle them.

Wellington's Justin Lester and other mayors had written to the Government calling for levies on plastic bags – but now questions are being asked of councils as to why they're not doing their bit and providing adequate recycling or composting capacity. Attempts by shoppers to be conscientious are being stymied by the lack of composting capacity.

Bags Not campaign boss Nick Morrison called for hundreds, even thousands, of rat-proofed, leachate-free hot composting sites to be set up across urban areas.

He said most compostable coffee cups and bags were ending up either in landfill, where they could not break down, or contaminating plastic recycling streams. Some of the products could not be composted because they were not considered to be organic.

New Zealand sends 1.6 billion single use plastic bags to landfill every year, he said. "The Government can really show some leadership. Setting up composting infrastructure and putting some effort and resources into educating the public. There is so much confusion. People don't know what is biodegradable, degradable, compostable, recyclable," he said.

He also called for the Government to increase the landfill levy from $10 a tonne to something more in line with the $150 a tonne in the UK and Australia.

Both Morrison and Farrelly said the most likely solution to single-use plastic bags was first a levy, then a complete ban.

Councils and the Government should embrace the shift in public mood shown in campaigns like Bags Not and Plastic-Free July, Farrelly said.

* An earlier version of the story said The Warehouse bags will not be able to be composted in household bins because they are not big enough to produce the heat required. However, The Warehouse has confirmed its bags are certified for home composting.