Worried environmentalists have demanded the release of the full study so water consumption can be monitored better.

Conducted between the middle of last year and early this year, the audit examined water consumption at five pool complexes in Fanling, Kowloon Park, Kowloon Tsai, Morrison Hill and Sha Tin. The pools use 1.3 million litres of water every day, according to the report.

The outdoor Sha Tin Jockey Club pools are the most wasteful, according to the report, using an average of 1,310 cubic metres per day. That includes water used in showers and toilets as well as water for replenishing the pool, and filter backwash and cleansing.

That's the equivalent of half the water in an Olympic-sized pool being wasted every day and based on a standard non-domestic charge of HK$4.58 per cubic metre, the cost would be HK$6,000 per day.

The Sha Tin complex consists of seven pools, including a main pool, a secondary pool, one training pool and two teaching pools. But only 990 people swam there on an average day in the peak summer swimming season in 2010.

The means every visitor to the complex used an average of 1,323 litres of water on every visit - about six times the daily amount of fresh water a Hongkonger uses.

According to best practice guidelines issued last year by the Australian utility Sydney Water, a swimming pool complex's water efficiency is considered to be poor if the average water use per swimmer is more than 60 litres. Best practice is to use less than 20 litres of water per swimmer, the utility said after studying more than 40 aquatic centres.

The Water Supplies Department declined to comment on whether the usage was above the international norm.

'It is not appropriate to benchmark the usage against [that of] overseas, since the water consumption level varies with service levels, operating environment, patronage rate, design and maintenance requirements,' a spokesman said.

At Morrison Hill in Causeway Bay, there are just four pools and only 300 cubic metres of water is used daily on average, although an average of 2,200 people swim there. It is a stark contrast to the Sha Tin pools.

Brian Lam Wai-ping, a swimming pool contractor with experience in public pools projects, said the usage figures were exceptionally high.

'If the figure is accurate, I am very surprised and puzzled by that. How can that be?' he said.

Lam said many outdoor pools, like those in Sha Tin, suffered due to water evaporation. A poorly maintained pool could have leakage issues too, he added.

While the Water Supplies Department refuses to break down how the water was used or lost, the Sydney utility's audit of 10 pools there last year found that pool replenishment and filter backwash generally accounted for about 36 per cent of water used.

About 22 per cent is lost to leakage while 35 per cent is used in the showers and toilets.

Chan May-ling, chief executive of Friends of the Earth, called for a full disclosure of the audit results. 'This will introduce a healthy competition among the pools to reduce consumption,' she said.

Cheng Cho-kwong, chairman of Sha Tin district council's sports and community development committee, also urged the pool's operator to 'study why the water consumption is at such a level'.

The Leisure and Cultural Services Department said the audit yielded some recommendations for improving the water efficiency at the pools, including regular monitoring, the replacement of inefficient designs and practice, the replacement of aged filters and use of water jets instead of hoses for poolside cleaning.

The Water Supplies Department said it was developing a best practice guide for pool operation.

Last year, about 9.5 million people visited the city's 41 public swimming complexes. Estimates from the audit suggest at least 6 million cubic metres of water are used by these complexes on an annual basis, constituting half of the 13.5 million cubic metres used by the Leisure and Cultural Services Department.

41

The number of swimming pools operated by the Leisure and Cultural Services Department