The eye-watering odours wafting out of the Sydney Water North Head Sewage Treatment Plant and the trucks that cart off the excess solid bits (treated poo) through the seaside suburb of Manly have for years plagued residents, businesses and stunned unwitting tourists.

"It is so bad on some days, " said Dr Geoff Lambert from the North Head Sanctuary Foundation (NHSF), "it makes you want to vomit."

Dr Lambert said when the trucks carrying treated poo leave the plant and travel through the streets, the smell literally has people wilting in its wake.

The NHSF secretary Kathy Ridge said there have been days when the stench is so bad she has had to leave the office, and an employee from another nearby group has quit her job.

But now scientists across the world have been called in to work on solutions that they hope will fix the stink once and for all.

Sydney Water, the owner of the sewage treatment plant, has engaged scientists from the French company Suez Environnement and they are busy testing the waste water in their research and development facility in Paris looking for ways to reduce and eliminate the stench.

Nicola Nelson, Sydney Water's manager of health, environment and asset science, said:  "It is the first time we have done this at such a sophisticated level and with processes underpinned by science."

A spokesman for Sydney Water said it is spending $1 million a year for  four years to attempt to find what is causing the problem and to find ways to stop the offensive odours. Some of the work already being undertaken includes cleaning and refurbishing the plant's equipment and and installing new biotrickling filters and installing new sensor equipment to detect odours. He said they are hoping to get a much better picture of the odours being emitted and how to predict where and when they might occur in the future.

Robert Kelly, the Director of Technical Assistance for the Wastewater & Environment Division of Suez, said it was also encouraging neighbours, residents and workers in the area to be "educated" in how to identify the type of pong with an "odour wheel"  giving them descriptive words like burnt match and rotten eggs to help them track down the culprit process or material that is causing the problem.

Manly Independent councillor Candy Bingham said that the trucks taking the treated solids (poo) away from the plant to be used as fertiliser, travel through residential, commercial, and tourist areas leaving a "trail of pong as they go which lingers for a long time".

Greens councillor Cathy Griffiths said the plant was polluting the air of one of Sydney's most valued tourist attractions and it was about time it caught up with the rest of the world in dealing with sewage.

The Environment Protection Agency is reviewing Sydney Water's North Head operations and submissions closed earlier this month.

Natalie O'Brien visited the Paris research and development facility courtesy of Suez Environnement.