IT TAKES a lot to shock Dave Thomas, a diver who has worked in conservation for more than 20 years.
But as he walked to work on January 2, he found an unusual souvenir from 2012 - dozens of plastic fireworks cartridges washed up at Chowder Bay on the north shore.
The vivid-green containers rained from various displays on New Year's Eve, then they floated around the harbour until they were retrieved by Mr Thomas, who describes himself as a ninja conservationist.
''I've been doing this for so long, nothing surprises me,'' he said. ''I was out on the harbour for all of the fireworks, and they were spectacular, but people don't think about what happens afterwards.''
Seven tonnes of fireworks were used over the night, comprising 11,000 shells, 25,000 shooting comets and 100,000 individual pyrotechnic effects.
The provider, Foti International Fireworks, has supervised the New Year's display for 13 years, and says it is a carbon-neutral company that uses ''low-environmental-impact fireworks and recycling''. The fireworks director, Fortunato Foti, said the plastic was part of a Roman Candle firework that divided into different sections.
He said the part had been changed by the manufacturer, and 200 to 300 fireworks in their display contained it.
''We've already made inquiries to the manufacturer,'' he said. ''They're usually made of natural products, we've never had this issue before. It's a common firework that's used, and it's a pretty common component.''
A spokeswoman for the City of Sydney council said it was investigating how this debris ended up in the harbour without its knowledge.
''The City was unaware and regrets that a specific type of firework used in last year's display had several components that were not biodegradable and these entered the harbour,'' the spokeswoman said.
''We have instructed the contractor that these pyrotechnic effects are never to be used again.''
But Mr Thomas said there's plenty more waste to be found in the harbour. ''Seventy-five per cent of rubbish is under the surface, so it's part of a wider waste problem,'' he said.
Morning & Afternoon Newsletter