Up to 35,000 cigarette butts are washed into the Hauraki Gulf each day, making them one of the worst polluters of Auckland's swimming spots.
Groups who cleaned Auckland's harbour calculated that one in three cigarette butts ended up as litter, many of which made it into the sea through stormwater drains.
Given the size of Auckland's smoking population, this meant about 35,000 cigarettes ended up in the ocean.
The estimates come a day after volunteers revealed they were fishing up to 3500 litres of rubbish from the shores and waters of the Hauraki Gulf. A one-day December clean-up around Rangitoto collected 200,000 pieces of rubbish.
Environmentalists said the thoughtless dropping of cigarettes was a good example of how people's behaviour needed to change.
"A lot of people don't even realise that cigarette butts are rubbish. They get flicked straight on to the street," said Sustainable Coastlines head Sam Judd.
"You can walk down Queen St and open any drain marked with 'Flows to the Sea' and it's just full of cigarette butts."
Care For Our Coast, a clean-up programme run by the Sir Peter Blake Trust, picked up 1000 used cigarettes at its most recent event at Mission Bay.
The litter was in excess of what council contractors at the site could deal with, said the organisers.
Smokers' litter has increased significantly since the indoor smoking ban in 2004.
Cigarettes contained toxic ingredients which released slowly into the marine environment and took a long time to break down.
Although larger objects were also found in the harbour, the smaller items were the most difficult to collect and were the most likely to harm wildlife.
Mr Judd said no amount of regulation, street cleaners and volunteer projects could contend with the amount of minuscule litter making its way through our waterways.
His group worked to clean "nurdles", or small teardrops of plastic in its rawest form, from rivers before they reached the ocean.
"Once [this] rubbish reaches the coastline in this state, it is impossible to clean up."
Auckland Council environment and sustainability forum chair Wayne Walker said he felt the amount of litter in the city's harbours was getting worse.
He said the council, like volunteer groups, was focused on behaviour change. Plucking rubbish from the sea was expensive and labour intensive.
"The main message we need to get across to people in Auckland is that they must take responsibility themselves. It's an individual problem," said Mr Walker.
The council is reviewing its waste and recycling strategies and a report will be discussed at the forum's first meeting of the year next month.
Mr Walker said some of the proposals being considered were putting pressure on industry to use environmentally friendly packaging and possible fines for heavy polluters.
As the Rugby World Cup approaches, projects are already under way to improve the cleanliness of the central city and water's edge.
The Beautify Your City campaign has encouraged more responsible disposal of waste by restaurants and shop-owners.
Rubbish collection times in the central city are strictly limited to an hour to reduce the amount of time rubbish bags sit on the street.