New South Wales residents have returned more than 900 million cans and bottles since a container deposit scheme was introduced in December.

The idea is simple: get 10 cents back for every eligible can or bottle recycled at one of the 800 reverse vending machines around the state.

It is hoped the scheme will reduce litter in NSW by 40 per cent by 2020.

National Marine Science Centre director Steve Smith, who has been part of a series of statewide surveys that measure the amount of plastic rubbish ending up in coastal estuaries, said before the scheme was introduced, there could be up to 300 plastic items found in every 1,000 square kilometres in Coffs Creek.

"It's not all big items either. Our studies [around the mid-north coast and Coffs Coast] have been focused on items that are larger than 5 millimetres, but there's a lot of hidden plastic, like micro-plastics, as well," Professor Smith said.

"Styrofoam is one of the worst offenders, but we're finding plastic food packaging and glass fragments too."

The amount of cans and bottles returned is expected to reach 1 billion in December, the NSW Government says. ( ABC News: Sarah Maunder )

Professor Smith said there had been less plastic rubbish found since the scheme was introduced.

"There is a trend downwards, which is fantastic," he said.

"Maybe it's not as dramatic a reduction as we would have liked, but overall across the state, if we compare September 2017 to September 2018, we went from an average of 20 containers per 1,000 metres down to around six."

A way to earn extra money

For most people, the Return and Earn reverse vending machines are a creative way to earn some extra cash.

Jayce Cameron, 12, from Port Macquarie, has been collecting bottles and cans for six months.

He accidentally broke the windscreen of his dad's car, and is using the money he gets from Return and Earn to help pay off the repairs.

"We went to the Bathurst V8s this year and I kicked the windscreen while I was sleeping," he said.

"After the race, there were all these cans and bottles everywhere, so we collected it all. I think it will help pay off about a 10th of the windscreen."

Rebecca Tighe, of Port Macquarie, whose son Bradley also uses the scheme to earn money, would like to see more items able to be recycled.

"We've been using Return and Earn for a few months. We really like it. I just wish you could return wine or milk cartons," she said.

"You're pretty limited in what you can return."

Bradley Tighe, of Port Macquarie, is one of many young people using Return and Earn as a way to get extra pocket money. ( ABC News: Sarah Maunder )

Scheme having positive impact, Minister says

A spokesperson for NSW Environment Minister Gabrielle Upton said the Return and Earn scheme had been a great success.

"More than 930 million drink containers have already been collected across NSW return points," the spokesperson said.

"This includes more than 6.4 million containers collected in the Port Macquarie area, and 14 million collected from the Coffs Harbour region.

"This demonstrates the immediate and positive impact that Return and Earn is having on litter reduction in NSW, and ensures we are on track to achieve the Premier's target."

So far Victoria and Tasmania are the only Australian states or territories yet to introduce a container deposit scheme.

South Australia has had one in place since 1977, and the state's Environmental Protection Agency said it was as popular as ever, with a return rate of almost 80 per cent.