ACT leader David Seymour's claim that reusable bags could breed harmful germs has been backed up by a food safety expert.
Single-use plastic bags are being phased out by the Government over the next year, Prime Minister Jacinda Ardern announced in August, to "safeguard New Zealand's clean, green reputation".
But Mr Seymour says there are risks to people reusing bags to carry grocery items like raw chicken, and up to 20 people could die each year as a result. With summer coming up, Mr Seymour told Newshub food-borne illnesses could spike as people pack raw food into bags and reuse them.
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"You can see that happening in the Kiwi culture over summer - hot cars, lots of barbeques; and it makes the wider point that we seem to be facing an environmental policy that is almost based more on emotion than science," he said.
"I'm not necessarily opposed to the plastic bag ban. But if the Government was able to show what the likelihood that a plastic bag someone takes from the supermarket ending up in the sea is, and it was a high probability, then it might be the right thing to do to ban plastic bags."
Mr Seymour's comments are based on a 2013 George Mason University study of the 2007 San Francisco plastic bag ban. It found that admissions to hospitals in the area related to food-borne illnesses had increased following the plastic bag ban.
The ACT leader's concerns over reusable bags have been backed up by food safety expert Dr Steve Flint. The Massey University Professor told Newshub the main problem is with products such as chicken. But any raw food has potential for food-borne pathogens to leak out, particularly if it's loosely wrapped.
"The contamination that comes out of that product will end up contaminating the bag, and if it's a cloth bag, it'll absorb that material, and the food-borne pathogens can stay in that bag for quite a period of time," he said.
The bacteria campylobacter has been an issue in New Zealand for many years, associated mainly with chicken. The amount of incidents has been reduced by half, according to Dr Flint, but he said it's still a concern, especially now that people are reusing grocery bags.
"It's a pretty severe disease," he said. "I was working with a guy in his 20s, a pretty fit guy, and he was knocked back for about six weeks by it. We eat a lot of chicken, and the way it's wrapped in many supermarkets is not in a sealed plastic bag, so often liquids leak out."
The other risk would be salmonella, he said, which is also associated with chicken and other meats. An organism like salmonella "will survive even if the bag is dry for a long period of time," he said.
"With the plastic disposable bags, it wasn't an issue, because the bag was generally used once and then thrown out. And even if it was used again, the plastic surface isn't going to absorb anything which means there's less likely to be a source of contamination."
In terms of trying to manage it, Dr Flint recommends washing reusable bags on a regular basis, particularly if you've used them for raw foods. The other possibility is using separate coloured bags for raw food and the other for food that you're going to eat straight away.
Associate Environment Minister Eugenie Sage said in August that the Government will work alongside supermarkets and other retailers to help people make the change to reusable bags. She was confident New Zealanders would embrace the change and get used to the ban on single-use bags.
"Public calls for action have encouraged a significant number of retailers, including supermarkets, to move on single-use plastic bags," she said. "We want to support their efforts by ensuring the retail industry moves together in a fair and effective way."
But Mr Seymour says plastic bags "actually deliver a lot of benefits and no one has really weighed up those benefits against the costs, because they don't know what proportion of plastic in the ocean is coming from New Zealand". He said he "suspects it's almost none".
"I consider myself a real environmentalist - I think environmentalism is important but I think it requires science, it requires economics, and it requires weighing up difficult questions."