Each person in the city uses an average of 130 litres of fresh water every day, which is higher than many other major cities across the world, including Singapore and London.

"We would need to develop more water resources while cutting our water consumption so that we can be in a better position to tackle any uncertainties that may arise in the future," he wrote, referring to the shrinking global supply of fresh water due to world population growth, economic development and climate change.

He said the Water Supplies Department had been exploring new sources of fresh water supply - such as desalination, extension of the seawater for flushing supply system, sewage recycling and rainwater harvesting - but the government had to rely on the public to curb demand.

As well as the fresh water usage, Hongkongers use an average 90 litres of flushing water every day, which adds up to the current total of 3.3 million litres of water consumed in Hong Kong every day - enough to fill the main pool at the Victoria Park Swimming Pool complex 1,100 times, he wrote.

"To adjust our daily habit in water usage does not mean we will go back to the days when water supply was restricted," he wrote. "Water saving can be easy without sacrificing the quality of life."

"Avoiding leaving the water running when shaving and brushing teeth can save a family of four 26 litres of water per day; or soaking fruit and vegetables instead of rinsing them under running water can save another 14 litres of water a day."

He called on the public to join the "Let's Save 10L Water" campaign, which the Water Supplies Department launched last Saturday as a first step for households to get involved in better water-saving habits.