New Zealand's environment watchdog has asked the Government to clear up confusion surrounding biodegradable and compostable plastics touted to be cleaner and greener.

Parliamentary Commissioner for the Environment Simon Upton's call comes as more retailers move to ditch conventional single-use plastic bags for such alternatives.

While there were many supposedly environmentally-friendly plastics coming onto the market, it remains extremely difficult for consumers to make sense of their respective claims, Upton said.

The main problem was, he explained, that whatever impact biodegradable plastics could have on the environment, very much depended on what they are made from and, critically, how and where they were disposed of.

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There was a risk some companies' claims might amount to little more than greenwash.

"There is a surprising degree of complexity at play here," Upton said.

"One can't simply toss these products onto the compost heap, or into our recycling bins, and go away thinking 'job done'."

His office this morning released a paper explaining the technical aspects of biodegradable and compostable plastics.

Biodegradable essentially meant that an item could be broken down by the action of living organisms, typically microbes.

While New Zealand didn't yet have a standard for biodegradable or compostable plastics, some plastic manufacturers had voluntarily sought certification using some of the existing international standards so that they can label their products accordingly.

Products that were certified compostable, meanwhile, would usually have been tested in commercial composting conditions, and would not necessarily biodegrade in a home compost heap.

Some of types of plastics were sourced from biologically produced compounds and could be broken down by microbial action.

They included polylactic acid (PLA), sometimes used in food packaging and coffee cups; poly (hydroxyalkanoates) (PHAs), commonly used in disposable cutlery; and thermoplastic starch (TPS).

Plastics classed as "oxo-degradable" were sometimes marketed as biodegradable, but shouldn't be unless they passed existing international standards for biodegradability.

Some other bio-based plastics, such as PET, generally still weren't biodegradable despite being manufactured from at least some biologically produced compounds.

Whether biodegradable or compostable plastics were actually better for the environment than conventional plastics still depended on a range of factors.

If littered, even certified biodegradable plastics might not break down in the way expected.

Compostable or biodegradable plastics could contain additives and questions remained as to whether currently available standards adequately addressed these.

There were also questions around the climate-change implications of breakdown of biodegradable and compostable plastics.

Upton has written to Associate Environment Minister Eugenie Sage asking the Government to urgently sort out headaches around terminology, to explore standards and labelling, to review what facilities were in place to receive the products, and to clarify how the products fitted into environmental goals.

He also suggested new research looking at the impacts of trade-offs between different material types and disposal options, whether incentives for businesses and consumers were aligned with the most effective solutions, and what risks were posed by potentially toxic components within plastics.

"I am pleased that Minister Sage has acknowledged the issue, but there is a bigger role for the Government to play in providing much-needed guidance and regulation," Upton said.

"The array of environmental claims being made about plastic packaging can lead to misunderstandings on the part of even the most environmentally conscious citizens."

Waste Management Institute New Zealand chief executive Paul Evans agreed there was "significant confusion" among retailers and consumers around terminology.

"Many consumers have purchased bags thinking they've bought something compostable only to find out it's actually degradable - so sorting out these terms is actually very important," Evans said.

"That's why standards and labels are so vital in allowing consumers to make informed decisions.

"At the moment there are a plethora of standards and manufacturers are creating their own labels, which only adds to the confusion."

Evans argued that no single-use option was a good choice.

"There may be some which lessen the environmental effect to some degree - but ultimately they all have an impact."

Sage agreed work was needed.

"It's very hard for families and businesses to make choices that are good for the environment when some products don't do what they say."

The resources released today fitted well with wider work she was leading.

"Greenwashing is a real problem for people trying to do the right thing and I want standards implemented for biodegradable and compostable plastics and the Ministry for the Environment is looking at options for a labelling or certification regime.

"The Ministry for the Environment is also already working with industry leaders to develop a voluntary minimum standard for biodegradable and compostable plastics."

Sage noted Upton's call for a review of end-of-life infrastructure for biodegradable and compostable plastics.

"We are already reviewing onshore collection and processing services as part of our response to the current pressures on New Zealand's waste and recycling sector," she said.

"We need to design waste out of our economic system and create and buy products designed to have a long life, which can be easily disassembled so they can reused, recycled or composted."