The NSW Government has mandated the ban on all free - and paid - polyethylene bags of 35 microns. A micron is one thousandth of a metre and is used to measure the thickness of plastics.

Lightweight bags made from biodegradable or compostable plastics, or bioplastics, will also be prohibited as they can only be broken down at compost facilities.

Produce and deli bags, bin liners, human or animal waste bags, and bags for medical items will be exempt from the ban.

The ban became law through the Plastic Reduction and Circular Economy Act 2021 - passed by NSW Parliament in November 2021 to curb environmental damage from plastics.

NSW environment minister James Griffin said it marked the start of a major pivot away from single-use plastics for the state.

"All of us can see the impact plastic pollution is having on our environment, which is why we're making major changes in NSW this year," he said.

"Single-use plastic is used by many of us for just a few convenient minutes, but it remains in our environment for many years, eventually breaking into microplastics.

Single-use plastics made up 60 per cent of all litter in NSW, Mr Griffin said.

Next on the agenda is single-use plastic straws, bowls, plates, and cutlery, polystyrene food containers and cups, and personal care products containing plastic. These will be banned from November 2022.

Sole traders face fines up to $11,000, and businesses up to $55,000, if they are found in breach of the law.

Larger companies, including manufacturers, producers, wholesalers and distributors could be slapped with a $110,000 fee.

Shoppers will be encouraged to bring their own shopping bags. But stores can offer alternatives, such as paper bags or heavy-duty plastic bags - at a price.

Supermarket chains Coles and Woolworths phased out their single-use plastic bag supply in 2018 and have since sold a range of canvas, jute, plastic tote and chiller bags at their stores.

They also offer customers heavy-duty plastic reusable bags for 15c.

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Monash University-led BehaviourWorks Australia research fellow Mark Boulet said the promotion of heavy-duty plastic bag options may have similar consequences as lightweight bags if not well regulated.

"Retailers and consumers may simply change to using an alternative product which has the same environmental impact; it's just using a different type of plastic," he said.

"They might charge [for thicker plastic bags], but the charge is often so low that consumers eventually just end up absorbing the cost.

"I see people very happily buying up to a dozen bags, and you can just tell that they'll probably end up doing the same thing again in a couple of weeks when they do their next big shop."

NSW will be the last jurisdiction in the country to implement a lightweight plastic bag ban.

South Australia was the first in 2009.

It was followed by the ACT and the Northern Territory in 2011, Tasmania in 2013, Queensland in 2018, and Victoria and WA in 2019.

All states and territories prohibit the same types of bags - 35 microns-or-less - but the ACT, NT and SA have excluded biodegradable bags from their bans.

Mr Boulet said a BehaviourWorks review of plastic bag bans across Australian states and territories was so far inconclusive.

"There's nothing really robust that they've been able to gather in the last six months to a year, where they've been able to measure the effectiveness of these approaches in terms of either changing behaviour or single-use plastic consumption," he said.

He said United Nations and Organisation for Economic Co-operation and Development (OECD) reports on similar bans abroad, including in the US, Europe, Asia and Africa had revealed some positive effects.

"These approaches actually do see, in the short term at least, a reduction in the particular item that they're targeting," he said.

"With the caveat that the data sets are not always that great ... there's a growing body of evidence to suggest that these bans are effective."

But Mr Boulet warned a lack of enforcement could cause "a slow creep back to the status quo".