Campaigners fighting plans to build a man-made beach at Tolo Harbour say official data shows the water in the area is still unfit for swimming.
An indication of the actual situation came to light only after they put pressure on the government to release the hidden figures behind its announcement last month that tests showed "an improving trend".
The data, collected at three spots along the existing natural Lung Mei beach between 2010 and January shows that the concentration of E coli bacteria was 1,600 per 100 millilitres of water, which is considered "very poor" compared with a safe level of 180 per 100ml.
However, those figures indicate average water conditions over time, a measurement known as the geometric mean which evens out fluctuations.
Actual concentrations of E coli - which is associated with sewage and food poisoning - recorded at certain times were therefore likely to show even higher concentrations.
In another set of government figures handed over, the level is lowered to 180 by further evening out fluctuations.
Peter Lee Siu-man, campaign manager of the Conservancy Association and a spokesman for the alliance of 15 groups fighting the beach plan, said: "We accept that the standard methodology was used, but we still urge the department to publicise the actual measurements, and not the mean, so we know how dirty the water can get on the dirtiest days and the cleanest days."
He added: "To us, the data is merely showing cyclical changes in water quality, not a trend of improvement. It is misleading to say the water quality is getting better."
The alliance has campaigned against the artificial beach at Lung Mei in Tolo Harbour for years, arguing that dumping sand along the coast to create the beach would wipe out hundreds of marine species. Tai Po District Council, which covers the site, supports the project.
The alliance also found that the environmental department had made no progress on a review of "marine water quality objectives", after a first stage of public consultation ended in 2010.
The department had, in the review consultation, considered revising the standards for recreational water environments to match World Health Organisation guidelines. That would mean measuring faecal streptococci - which can cause pneumonia, ear infections and meningitis - instead of E coli.
The Environmental Protection Department said its method for monitoring water quality was long-standing practice to reflect long-term trends, taking out short-term fluctuations.
The department added it had started to gather information on other bacteria at 41 public beaches back in 2011. The second stage of the review of marine water quality objectives would take place this year and had to wait for data to be collected and complex calculations completed.